I think full winters don't make that much sense anymore for UK motorists. If temps drop to -20C, then perhaps the All seasons will really start to drop off in performance. But for UK use I think the All-season's make better winter tyres than full winter tyres, if you feel the need to 2 sets of rubber; because most winter days and nights are not that cold anymore. Even in Lincolnshire I think I only had to de-ice the windscreen 6 times or so last year. (So much for my my heated windscreen); so when temps get above 5c/7c the winter rubber becomes pretty poor, if not blood dangerous.
The big advantage to me is the Mud and Snow (M&S) and Aquaplaning resistance as much as the winter use. Proper versatility.