Met Office UK's Met Office, includes aviation-oriented self-briefing data.
MeteoNet Reliable Synoptic Prognosis for next 5 days.
XC Weather Good visualization of UK TAF and METAR data.
MetCheck Not always 100% reliable, but offers an easy-to-understand forecast for the area.
MeteoBlue Good variety of forecasting information.
GlideMet Daily gliding forecast for the UK.
RASP Computer modelled simulation providing (amongst other things) a predicted thermal map of the UK.
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NOTAMs Not strictly a weather link, but provides Notices to Airmen with regard to temporary airspace etc.