I know A-levels aren't for everyone, but why are you considering this route to a commission? Do you have the GSCE grades for the officer role?
As for timescale, it could be several years, so would you be happy doing an airman's job for that long? Your early years as at junior rank in any branch of the Armed Forces can involve training courses, exercises, deployments, parades, guard and secondary duties; for the latter it can be difficult to find a position where you've got sufficient responsibility to show the potential for promotion, let alone applying for a commission. It's a repeat of the joining process, and it isnt enough just to be doing a good job day-to-day whilst you're in the Forces; that's meeting expectations, you need to exceed them and have the backing of your boss.