BBC Weather
BBC reports for the UK
http://www.bbc.co.uk/weather
Met Office
Detailed reports for all areas of the UK
http://www.metoffice.com
MET Office Mountain Area Forecasts
Specialist reports for the UK's mountainous regions.
http://www.meto.govt.uk/services/phone/prphonemo.html
Metcheck
This site has a section for hobbies including mountaineering, showing weather forecasts for most UK mountain ranges
http://www.metcheck.com/V40/UK/HOBBIES/mountain.asp
Online Weather
Weather Forecasts for the UK & Ireland complete with pages designed for your PDA or WAP phone.
http://www.onlineweather.com
Scotish Avalanche Information Service
Daily avalanche forecasts for the Scottish Highlands.
http://www.sais.gov.uk
Snowdon Weather Station
They offer you a year round window on the high mountains of Snowdonia. Complete with live WebCAM.
http://www.fhc.co.uk/cams.htm