The prevailing wind direction depends on the time of year. On average over the entire year the prevailing direction is from the southwest (approx 245 deg) over most of the UK. The Met Office have a couple of wind roses for the some stations in the southwest (strangely the only two I could find quickly) that show this:
Wikipedia helps the interpretation
Wind rose - Wikipedia, the free encyclopedia
Over the summer months the UK tends to be on the eastern edge of an anticyclone that sits over the Atlantic. This generally gives westerlies, but as it moves around there are blasts of easterlies or southeasterlies over the country, which brings in a lot of the muck from the continent (hazetastic!).
I wouldn't use climatology for any sort of flight planning (or should that float planning?). Looking at the surface analysis pressure chart for the day of will give you a reasonable indication of which way you'll be pushed as the wind tends to blow pretty parallel to the isobars at 1,000ft or so.
Here's a link to the latest UKMO surface maps:
http://www.wetterzentrale.de/pics/bracka.gif
Cheers,
Gareth.