www.metoffice.com
The metoffice website offers you an indepth forecast for a specific date, time, region and town plus a 5 day forecast. Their services include a specifically tailored weather report for your trip, regular updates to your mobile phone as they happen and support for shipping.

uk.weather.com
Another excellent weather site with a comprehensive guide to the current and forecast weather for towns and cities including an hour by hour forecast, a 10 day forecast and specific weather support to fishermen.

www.onlineweather.com
Todays weather at a glance but you can also check the outlook for upto 15 days ahead plotted out for you on a graph. The accuracy of any forecast/outlook beyond 5 days in advance is obviously going to be less reliable though.

www.xcweather.co.uk
Weather at a glance with details of wind speed, direction, visibility, pressure and temperature. Nice site.