studying for my AS levels at college (which I have to say is s**t) . . . if I decided to quit my AS levels now, would the RAF look down upon me as unmotivated to achieve,
It might send the messages that you are motivationally limited and academically challenged.
Should I look around for another studying centre and re-do my A levels somewhere else, but I'm 18 now and that would mean finishing around age 20-21 meaning I would be having to pay for education also
This would support your contention that your present college is poor, but unless you then produce the required A-levels, the first assumption would remain valid.
Now if you do switch and you do press on to 21 and you do pass that would indicate good motivation and go somewhere to showing you were not academically challenged./ At interview you would need to explain why you were 21 and not 18 when you got your A-levels. A well argued case could be a bonus.