Most forecasters state that there's little point trying to forecast the UK weather more than about 5 days ahead. (Some suggest that anything beyond 3 days is virtually guesswork). The most accurate forecast available is on the BBC website but it's limited to just 5 days:
http://www.bbc.co.uk/weather/
uk.weather.com attempts to provide a 10-day forecast but it's invariably more pessimistic (and less reliable) than the BBC one:
http://uk.weather.com/
I doubt that there's any site which attempts to provide a 14 day forecast. Roughly a week ago, uk.weather.com was forecasting cool temperatures and showers for the coming Saturday. Even now, they're saying that London will have showers (22C) on Saturday. The BBC site says that London will be sunny (24C) on Saturday. As stated, I have greater faith in the BBC site but it doesn't pretend to be able to forecast more than a few days ahead.
Chris