I would also add the Midland Gliding club at the Long Mynd (Church Stretton) to your list.
I would recommend that you go to one of the hill sites (Portmoak, Sutton Bank (Yorkshire GC) or Midland GC as above. As long as the weather is half decent you will get ridge and thermal soaring and possibly wave as well. The big advantage of a hill site is that the opportunities for soaring are increased, as even if there is no convection (thermals) the ridge is often working and you will be able to perform useful exercises and soar, whereas, if it is overcast at a flat site you will be straight up, straight down!
The only major difference between these is that at Portmoak the airfield is at the bottom of the hill and therefore probably slightly safer/easier as you do not have to regain ridge height+ to do a circuit. At the other two if you cannot regain the ridge then you are heading for a field at the bottom! (Been there, done that!)
I cannot recommend one of these sites above the others as I have flown on courses at all three of them and had a great time at all.