Met Office UK's Met Office, includes aviation-oriented self-briefing data.
WeatherCharts Reliable Synoptic Prognosis for next 5 days.
XC Weather Good visualization of UK TAF and METAR data.
Fen End Farm Weather Station Real-time weather and webcam at Cottenham.
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.
NotamInfo Not the official source of NOTAMs (so beware using this as your only source of information), but substantially easier to read.