Martin, despite what it says on the website you would not have much to do with aircraft or air operations as a painter and finisher. If you just want to touch aeroplanes, you can go to a museum. If you want to have a trade which will teach you useful stuff about aviation, consider
Flight Operations Assistant (Air Traffic Control). You will be directly involved in aircraft operations, and may become an Air Traffic Controller in time. If your GCSEs aren't adequate for this, you can join as a
Flight Operations Assistant. It's not quite as directly involved with aircraft, but you will be dealing directly with pilots in their flight preparations - and you can obtain your GCSEs in the RAF and later transfer to ATC.
Either of these trades will be useful should you subsequently manage to start training as a pilot, and are likely to be more fulfilling for an aviation enthusiast than Gunner or Painter & Finisher.
Scroggs