A very frequent topic of conversation hereabouts, and you'll get many different views
My personal favourite is theyr.net, for it's ease of use and accuracy
Have a look at Ant Veal's Weather Centre at www.greatweather.co.uk There's plenty to choose from there.
If you happen to be visiting Cornwall you will find www.destination-cornwall.co.uk has a great local weather forecast which is updated at 0700 daily.