Prestwick would be a good choice, both clubs get an excellent rep.I have trained at the Flight Centre and found the whole experience pleasurable.
There is also Glasgow Flying Club, based at Glasgow, and Edinburgh Flying club,obviously based at Edinburgh.
One comment, due to all the constraints placed on an individual whilst training, weather being the main one, it can be wise to choose a school where you are relatively close to the airfield. This can save wasted travel time and arriving at the field,to find the lesson cancelled because the weather has alterred/closed in.
Yes a call to the airfield to ask prior to leaving is prudent, however, sometimes this does not work. A small point, but from experience it can save a lot of wasted trips.