Map from Google - United_Kingdom

The climate of Great Britain, influenced by the Atlantic Ocean, is temperate and humid, with relatively small temperature variations between winter and summer. The weather, however, is variable, with frequent changes from day to day or even in the same day.

The average temperatures in the plains are above freezing even in January, when they hover around 39/41 °F in the main cities, and reach 43 °F in the south-western tip of the country; in July they range between 53.5 °F in northern Scotland and 65.5 °F in the London region.

Precipitation is frequent throughout the country, but is more frequent and abundant in the north and on the western slopes. For example, in the west of Scotland it reaches 60 inches per year, with about 200 days of rain (more than one day out of two!), while in Manchester, 31.5 in of rain fall in 141 days; in Plymouth, on the southwest coast, 40 in of rain fall in 142 days; the southeast is less rainy: in London only 23.5 in of rain fall in a year, distributed in 109 days.

Scotland map
In Scotland, the weather is very cool, damp, rainy and windy for most of the year.
Rainfall is more abundant on the western side and the highlands, where it even exceeds 60 inches per year, while it's less abundant on the eastern side, where 23.5/27.5 inches per year fall: so Glasgow is rainier than Edinburgh. However, the rains are common everywhere, and even in the summer months, there are more than ten days of rain per month on average.
Here is the average precipitation in Glasgow.
Average precipitation - Glasgow
Prec. (in)

The wind is more common in the western and northern side and in the islands: no coincidence that the main cities (Aberdeen, Dundee, Glasgow, Edinburgh) lie in the eastern or southern side, which are relatively sheltered. However, when the most intense low pressure systems occur, the wind can be strong everywhere.
The Gulf Stream makes winter fairly mild, at least compared with other countries at the same latitude: the average temperature in January and February is around 39/41 °F. However, its northern location makes it vulnerable to cold winds from Greenland, which can bring snow showers and frost, especially in the Highlands, but usually these periods don't last long, because the westerlies start to blow again soon.
In spring, the temperature rises very slowly, so that it's still cold in April, when the maximum temperature is around 50/54 °F. In return, spring is the relatively sunnier season.
Summer is very cool: the average maximum temperature in July and August ranges from 55.5 °F in the Shetland Islands, to 59/61 °F in the north-west, to 64.5/66 °F in Glasgow and Edinburgh. The minima at night are around 50/52 °F. In Scotland, even summer is a pretty rainy season, and the sunshine in mid-summer (July and August) is even rarer than in early summer (June).
Autumn is cold, windy and rainy. The wind in Scotland is frequent, and sometimes can be stormy, especially in autumn and winter.
Here are the average temperatures of Edinburgh:
Average temperatures - Edinburgh
Min (°F)343437394550525248433934
Max (°F)454650545963666663555045

In the Highlands of course the weather is colder, and in winter snowfalls often occur. The duration of snow cover is remarkable, because the temperature increases slightly in the other seasons. Even the wind in the Highlands is more intense and frequent, as generally happens in the mountainous regions.
Here for example the temperatures of Glen Ogle, which is located at 1,850 feet above sea level.
Average temperatures - Glen Ogle
Glen OgleJanFebMarAprMayJunJulAugSepOctNovDec
Min (°F)302830343943464643393430
Max (°F)373739455054575754464137

At higher altitudes the temperatures are even lower, as can be seen from the data of Cairngorm Mountain, at 4,085 feet, where there is also a ski lift.
Average temperatures - Cairn Gorm
Cairn GormJanFebMarAprMayJunJulAugSepOctNovDec
Min (°F)212123273036393936302723
Max (°F)302830363945504845393432

Loch Ness

The sea is not so cold in winter, at least if we consider the latitude, while it remains very cool, if not cold, in summer. Here are the average water temperatures in Aberdeen.
Sea temperature - Aberdeen
Sea (°F)464545454652555554525046

Wales map
In Wales the climate is cool and cloudy for most of the year, but it's milder in winter compared with Scotland, because it's more sheltered from air polar masses, but also from continental air masses which may affect England. The average temperature in January and February is around 41 °F. Summer is cool, especially in the northern part: the average in July and August is around 59 °F in the north and 61/62.5 °F in the south; the average maximum temperatures touch 68/71.5 °F in the southernmost cities located in the Bristol Channel (Swansea, Cardiff).
Here are the average temperatures in Cardiff.
Average temperatures - Cardiff
Min (°F)363639414652555552464137
Max (°F)464852576368727266595248

The rains in Wales are frequent throughout the year, and abundant in autumn and winter, while they reach a minimum between April and June.
Here is the average precipitation in Cardiff.
Average precipitation - Cardiff
Prec. (in)

The temperature of the sea in Wales is a bit higher than in Scotland, but still remains very cool, if not cold, in summer. Here are the average water temperatures in Swansea.
Sea temperature - Swansea
Sea (°F)504848505255596161595552

England map
In England, the climate is temperate, and it's cool for most of the year.
Here are the average temperatures of Liverpool, located in the west.
Average temperatures - Liverpool
Min (°F)363639414652555552464137
Max (°F)454548546164686663575046

In England, rainfall is more abundant in the north-west and in the south-west, where it exceeds 40 inches per year.
Here is the average precipitation in Liverpool, where it is at an intermediate level, given that it amounts to 33 inches per year.
Average precipitation - Liverpool
Prec. (in)

Winter in England is cold and cloudy, sometimes foggy, sometimes windy. During the winter, the average temperatures don't vary much from north to south: in January it's around 39/41 °F, in Newcastle upon Tyne as in Birmingham, and in the countryside around London. Even in Liverpool and Manchester, the average temperatures are similar.
In fact, the southernmost regions are the farthest from the pole, but they are also the closest to the European mainland, from which cold air masses of Russian origin can arrive during winter. For this reason, the east side (which includes London) is also more prone to snowfall. However, these snowy or frosty periods are typically short, because here too, as in Scotland, the westerlies come back with their mild breath soon.
If, as we have said the cold in Scotland comes from the north and in England from the east, it's not surprising that, in addition to Wales, the mildest area is south-western England (Devon and Cornwall), where the average temperature in January and February hovers around 43 °F (see Plymouth). Here snowfalls and frosts are rare.
Here are the average temperatures in Plymouth.
Average temperatures - Plymouth
Min (°F)393941434852555554484341
Max (°F)484852556164686864595450

On the tip of Cornwall, the island of Scilly and the islets in the English Channel (see Jersey) have microclimates in which the absence of frost allows the growth of certain species of palm. On the other hand, in the south-west, being so much exposed to the Atlantic currents, rainfall is more abundant.
Here is the average precipitation in Plymouth.
Average precipitation - Plymouth
Prec. (in)

Spring in England is very cool, and the temperature increases slowly; rainfall is still frequent, but less abundant than in autumn and winter.
Late spring is the sunniest period of the year, despite the almost daily presence of clouds; even the wind reduces its intensity and its frequency. In May, the country is covered with flowers.
June is a nice month: the days are long, the nature (thanks to the passion of the British for gardening) is in bloom, and the temperatures are pleasant. There are, however, as always, rains and showers.
Unlike in winter, during summer the temperature increases from the north-west to the south-east, so that the area of London, which is certainly considered quite cool for those who come from southern countries, is still the one with the higher temperatures. In summer, the daytime temperatures range from 64.5 °F in Newcastle, to 66 °F in Manchester, 68 °F in Birmingham and Cambridge, 73.5 °F in the London area. In the south-west, which during winter was the mildest area, summer is cool: the minimum temperatures are around 55.5 °F and the maximum around 68 °F.
Even in summer the weather is variable across the country, so that it can change from day to day, or several times during the same day. The south of England is the area most subject to hot periods, in which southern currents from Spain can bring a bit of Mediterranean summer, and the temperature may even reach 82/90 °F. These periods, however, do not occur every year, and usually last a few days.
During summer the rains are more frequent and abundant in the north than in the south: if in Manchester in July there are on average 12 days with rain, in London they are "only" 8.
Here is the average precipitation in London.
Average precipitation - London
Prec. (in)

While in the south-west (as in Wales) the summer, especially in the first part, is the least rainy season of the year (but this doesn't mean it's a dry season), elsewhere in this season precipitation is slightly more abundant than in spring, due to thunderstorms, which can be accompanied by wind gusts.
Here are the average temperatures of London.
Average temperatures - London
Min (°F)363639434854575752464137
Max (°F)464652576470737368615246


The temperature of the sea in England increases gradually moving southward, so that in August it reaches 59 °F in Liverpool and Newcastle, and 63 °F in the south, as you can see from the average water temperature in Brighton.
Sea temperature - Brighton
Sea (°F)484646485255596363615552

The best time to visit Britain is summer, from June to August: it's a very cool season in Scotland, and progressively warmer as you head south; it's quite rainy everywhere, but also relatively sunny.
In Scotland, in July and August there is an increase in rainfall and a decrease in sunshine: here the sunniest month is June, which, however, is cooler than the two mid-summer months.
In the whole of Britain, June is a good month: the days are very long, the country is in bloom, there's no shortage of rains and clouds, but they alternate with sunshine. Also May (which is very cool, and sometimes even cold, especially in the north and in Scotland), often provides an acceptable weather, at least in the second half.
The mid-summer season (July-August) in the center and south is pleasantly warm, and between the showers, the sun can come out.
As mentioned, the sea is cold even in summer: in August the temperature of the Atlantic reaches just 54 °F in the Shetlands, 55/57 °F in Scotland, 63 °F in the English Channel and in Cornwall.
Since the climate of Great Britain is not characterized by climatic extremes, there is not a season to be absolutely avoided, although it must be said that late autumn (November and early December) is characterized by very short days, mostly cloudy skies, frequent rains and sometimes strong winds, while the mid-winter period (January-February, and sometimes December) as well as offering mild days, can sometimes be quite cold.

What to pack

In winter: warm clothes, sweater, coat, wind jacket, raincoat. For Scotland and the Highlands, and for cold periods: gloves, hat, scarf.
In summer: clothes for spring and autumn, jacket and sweater, raincoat or umbrella. In the area of London, it's more likely that in some days you can wear light clothing, t-shirt and shorts.

For the climate of Northern Ireland, see the climate of Ireland.