In my book, the inshore waters forecast from the met. office is the most accurate, its main drawback is the short range nature, i.e. 48 hours maximum.
Otherwise I'd look at weatheronline
The FAQ on xcweather gives a very clear statement of the limitations of some of these site in that they are not moderated by forecasters with local knowledge and do not take any account of local effects like sea breezes and hills etc.
Sea breezes in particular can have a big effect on summer wind directions and strengths in the channel.