Don't aim low, set your sights on your real target from the beginning.
If you can make the entry grades for Technician why don't you try and go straight for Engineer Officer at entry? You could qualify for a university bursary. If you don't qualify for university entry then you may well find self study while serving as an airman too difficult. Even if you enlisted first, you will still need to get through the same OASC tests when you eventually apply for a commission. In my opinion, enlisting as an airmen and hoping to get a commission later is not a good proposition. Once you're in and trained as a technician why should they bother retraining you as an Engineering Officer?
Do your "A" levels and apply directly for a commission. In the meantime get involved in the ATC, do the D of E's award scheme, go Outward Bound - do anything you can that will help convince the OASC that you are good 'officer material'.