For years whenever I have been abroad and talking to foreigners about the UK I have told them that the best months to visit are May and September. To my mind they get the best of the weather, the midges aren't so bad, and being outside the holidays they are not as busy as the traditional June-August summer. I also prefer the cooler temperatures.
The last few years have given me reason to switch my recommendation to March / April, and even October. It's now April 18th. We had a new patio laid in our garden just over a month ago and we are yet to see what it looks like wet! Check out this website which lists historical weather data for a selection of weather stations around the UK. There's a definite pattern emerging where it is dry this time of year, before our 'BBQ Summers' degenerate into rainathons. Personally I love this sort of weather - it's warm enough to enjoy, but cool enough to still be able to be active without too much unpleasentness, and the rock is even cool enough for good frictional qualities making it perfect for climbing.