First thing to do should be to get a class one medical otherwise you will just be wasting time and money if you dont pass the medical.
I would then finish your GCSE's and A levels. What happens if you drop out of college do your atpl's and have a accidient say in five years time and lose your medical? you would be left with no career prospect and you should plan a fallback if it dosnt work out you have to remember this career path can be cut very short by the unexpected.
Assuming all goes well i would finish your a levels and do your ppl part time at weekends and during holidays. With A levels you also would be able to apply for any sponcership schemes that arise!
Judging by the fact you are aiming to finish at 22 I guess you are still young so i wouldnt worry about time. Im almost 22 and went through A levels and uni and now im only just about to start training but i feel safe in the fact I have other qualifications to fall back on.
Hope this helps, good luck whatever you decide
Chris