The UK is famously rainy, but the reality is more nuanced — it rains frequently but rarely heavily. London averages just 600mm of rain annually (less than Rome). The issue is unpredictability: bring a light waterproof layer regardless of the season.
Summers (June–August) are mild at 18–22°C, rarely hot. Winters (December–February) are damp and chilly at 4–8°C but rarely freezing except in Scotland. The best weather is May–September, with long daylight hours in summer. Scotland is cooler and wetter than England year-round.