Ideal for planning walks across the UK National Parks - the Met Office weather guides and forecasts that are now available for a number of areas including the Peak District, the Yorkshire Dales, Snowdonia, Brecon Beacons, Lake District and the Scottish East and West Highlands. These are not the same as their regional (postcode based) weather forecasts but provide predictions on weather conditions and potential hazards. Really useful and recommend bookmarking. Normal caveats apply!
See the Met Office website for further information and to check the weather conditions.
UPDATE: There's also the Mountain Weather Information Service that provides area forecasts for 5 areas of Scotland, Snowdonia, the Peak District and the Lake District.