I've had most success with reds in the Lakes around Patterdale. When I used to stay at the youth hostel they would come to the bird feeders outside the window.
In the north east I've also seen plenty at the Killhope Lead Mining museum. They have a bit of a red squirrel walk there.
Incidentally, when I was a kid they were at Hardcastle Crags near Hebden Bridge as well, but they have not been seen there for years now.
Chris