another vote for Brownsea Island-you need to sit and wait- patience is the key, not stomping around like a baby elephant with a point and shoot, that will get you no sightings at all
one I shot on Brownsea
DSC02490 by
Les Moxon Photography, on Flickr
Or try around the Keswick area in the Lake District - Dodd Wood, there is an RSPB feeding station there and plenty of Red Squirrels too, so long as youre prepared to sit and be quiet- drive along the A591 out of Keswick keep the Lake on your left, the wood is about 2 miles on your right - plenty of parking pay and display though
good luck
Les