I have to disagree. I've met quite a few pilots who don't have any fancy qualifications and were able to pass groundschool (some had even been out of education for years!) Wasn't easy but do-able.
Ok, I take back what I said about the theory being that hard.
Still, an A level definitely won't hurt and it leaves your options open for the future.
To the OP, you don't have to do 3-4 A levels, you can do as many as you want.
You can do 1-2 A levels that you are really interested in.