Though famous for its bad weather, London actually plays nice most of the time. Except in summer, you'll get clouds and rain, but apart from that it's a steady and bearable weather.
Best time to visit London is obviously summer, to enjoy parks, outdoor pubs and activities.
Do not go to London en Feb - March : It's raining, it's cold and the weather is changing many times a day.
Destination
When to visit London
Information about the weather in London, when to go and when to avoid.