A-levels are harder than GCSEs. Give yourself the best chance you can and try to study full time for them if you can.
As for moving from trade to branch (ie other rank to officer), there were several lads and lasses on my IOT doing the same thing (one of them was a Chief Tech who had seen it all and was a legend) and one of them was an ex-liney who went on to fly the Jaguar. That is certainly an option.
Joining the RAF Regiment- a dedicated bunch of individuals; if you are doing that as a way of getting in the door, choose again. Same goes for the Army. If you don't need to work for cash, then don't and focus 100% on your studies whilst broadening your CV which extra-curricular activities which will help you get past OASC first time around. Try DofE, a cadet organisation, part time weekend job, charity work, sports teams etc.
Finally, read the OASC sticky at the top of the threads page. It is a trawl to get the info you need/want but that is not even close to the chaffe you will experience to get into a fast jet cockpit. This forum tends to be harsh to those who have not done their own research.
Good luck back in the UK.
