Brighton, where it is

In Brighton, a coastal town south of London, in East Sussex county, the climate is oceanic, with quite cold, rainy winters and mild, relatively rainy summers. However, the southern coast of England where Brighton is located has a mild and sunny microclimate, compared to the rest of Great Britain.
In winter, during cold periods, the temperature can drop below freezing, and on colder winters, it can drop to -8/-9 °C (16/18 °F). It can sometimes snow as well. In summer, the temperature reaches 28/29 °C (82/84 °F) only in the hottest days.
The average temperature ranges between 5.5 °C (42 °F) in January and February to 17.5 °C (63.5 °F) in August. Here are the average temperatures.
Brighton - Average temperatures
Min (°C)3346912141412964
Max (°C)88912161820211815119
Min (°F)373739434854575754484339
Max (°F)464648546164687064595248

Precipitation amounts to about 850 millimeters (34 inches) per year, and is therefore more abundant than in London. The wettest season is autumn, along with the first part of winter. By February, precipitation decreases slightly in intensity and frequency, reaching a relative minimum in summer, when it still rains quite often. Here is the average rainfall.
Brighton - Average precipitation

In Brighton, the sun is rarely seen from November to February, while from May to August, it shines for an acceptable number of hours, and in any case, a little higher than in the rest of Great Britain. In an average year, there are about 1,900 hours of sunshine. Here are the average sunshine hours per day.
Brighton - Sunshine

The sea in Brighton is very cool even in summer, and may be too cold for swimming, at least for tourists coming from southern Europe or for those who suffer from the cold: it only reaches 17 °C (63 °F) in August and September.
Brighton - Sea temperature
Temp (°C)98891113151717161311
Temp (°F)484646485255596363615552

Brighton Marathon

When to go

The best time to visit Brighton is the summer, from June to August, since it is the warmest of the year, although even in this season, it's better to bring a sweatshirt, a light jacket and an umbrella. The month of June is a bit cooler, so you may prefer July and August. September is a good month as well: it's a bit rainier than the summer months, but the temperatures are still pleasant, especially in the first part.
As previously mentioned, the sea is very cool even in summer, however, on sunny days, when the breeze is not too cool, you can sunbathe.