Here I've tried to bring together all the weather services I can think of - if I've missed your favorite, please contact me with the details and I'll try to include it.
All the forecast links default to my local flying site, Richmond Park (except XC): just enter your postcode to find your local forecast.
XC Weather - Considered by many to be the most reliable wind predictions in the UK. Just hover over the arrow closest to your local flying site; click for more in depth detail.
Metcheck - Another well regarded service.
BBC weather - Good old Beeb, always trying to improve their weather service (and sometimes take a couple of steps backwards in the process)!
UK Met office - Reliable forecasts but on a cluttered, confusing page - enter your postcode bottom left.
Windfinder - I guess it has to be here, but in a way this is my least favorite wind forecast site - it's perhaps the best for global forecasts, but I've never found it very reliable.