PPL exams are no big deal, anybody who has or expects to pass a couple of O levels can do them with very little effort so I don't understand why there should be any difficulties doing PPL studies at the same time as A levels, the only problem arises if one is unable to devote enough time to the A levels.
So far as which schools, use the search function on PPRuNe that is especially useful if you plan to save time and money by doing your training in the US. If not, there is a good school at Elstree, can't remember the name but a search on Elstree will probably find it. If you live closer to another airfield, do a search on that name also. NEVER PAY UP FRONT FOR FLIGHT TRAINING. Schools go bust with alarming regularity.
If you are going to the US, use an Immigration approved school, using a non INS school and just telling the INS that you are there for a holiday is fine until you get caught and deported then try entering the USA again.
Good luck
