I'm not sure if you are too young - university is certainly not a bad idea these days anyway, but you will obviously have to weigh up 10K going to uni, against the 40-50K training costs for the ATPL. It is better to be younger rather than older though, as some employers will require less experience the younger you are (to a point.)
Your A level grades are not as important as your aptitude as a pilot - which it sounds like you have, and knowing that you enjoy it and have the comitment to see the training through. Your time in the ATC idicates this is the case.
It may be worth doing some more aptitude tests that can be done with various flying training organisations that are mentioned on these forums or with GAPAN (see their website
www.gapan.org). Also get a class 1 medical - about £400 to check you are fit enough before paying anyone else your money!
I am 22 and am hour building at the moment, I went to university and am glad I did (but I am fortunate enough to have some of the funds required for ATPL training.) You must decide what will work best for you.
Good luck,
TCM