For those of you in the UK, the DTI has a searchable wind speed database showing the average wind speed for your grid reference.
They also link to a location converter, so as long as you know your postcode you can find your grid reference.
Wind speed database
Speeds are given for 10, 24 and 45 metres above ground level. In my case, it's 6.1, 7.1 and 7.8 metres/sec respectively.