As someone with reasonable experience of both powered and unpowered flight (Silver C gliding / PPL (pending CAA approval - thanks BEags!) and RAF EFT powered) I would agree with the previous postings that gliding tends to be much, much much more sociable.
Some UK gliding clubs are weekend only affairs, some are 7 day a week operations. For winter flying I would definately recomend a ridge site, as this will allow longer flights. I can recomend Bristol and Gloucester (Nympsfield, near Stroud), Yorkshire (Sutton Bank, north-east of York), Midland (Long Mynd, near Church Stretton) and Scottish Gliding Union (Portmoak, near Perth) as excellent from my personal experience, although I'm sure there are many more. These sites all offer good winter soaring conditions amid awe-inspiring, beautiful country surroundings. Most of these will have instructors available throughout the week, although courses may be restricted to the summer months.
While Lasham does have a good reputation, I have to admit being fairly unimpressed by my experiences there personally. Costs are very high comparitively, and the massive membership may actually be a disadvantage. As previously mentioned, it's flatness may make winter flying somewhat dull. Be prepared to pay dearly for their "aerotow to solo in a week" course, which also deprives you of the buzz of winch launching.
Any questions do please feel free to PM me.