AirTEXT
London's air pollution alert service.
http://www.airtext.info/

BBC.co.uk
London weather, including local weather cams.
http://www.bbc.co.uk/london/weather/

BBC.co.uk
Weather information galore.
http://www.bbc.co.uk/weather/

Lonely Planet
Advice on when to go and what sort of weather to expect.
http://travel.yahoo.com/p-travelguide-742883-london_when_to_go-i

Met Office
Provides official weather forecasts for London and the rest of the UK.
http://www.met-office.gov.uk/

Metcheck
Detailed weather reports covering the UK and Ireland.
http://www.metcheck.com/

Smooth Hound Systems
Advice on the best time of year to visit London.
http://www.s-h-systems.co.uk/tourism/advice3.html#when

The Weather Channel
Check out the weather forecast for London for the next 10 days.
http://uk.weather.com/weather/local/UKXX0085

This is London
Details the average monthly rainfall, temperature and hours of sunshine in London.
http://www.thisislondon.com/myday/weather/