When you do a PGCE you are technically qualified to teach from 11 - 18 although, as correctly pointed out, most of your training will be aimed at the 11 - 16 range because we need more high school teachers. Once qualified, youll have QTS which means you can teach in any maintained school in the country. Technically, colleges (as well as independent schools) dont have to employ people with QTS, so in theory you could get a job in a college with just a degree. In reality, this is unlikely unless you have significant work experience already and if they did they would probably try to employ you as an unqualified teacher (meaning about 4k/yr less pay). For these reasons Id urge you to stay away from any sort of PGCFE (a PGCE for FE colleges etc) because it doesnt give you QTS and youll almost certainly receive poorer training and less pay by the empoyer.
The correct way is to do a normal PGCE as already stated as this qualifies you to teach the entire age range and then apply fr a job at a sixth form college. However, as a word of warning, I know my head of department would (not uncommonly) refuse to employ an NQT as an AS or A2 teacher - you would need a couple of years experience first and that means high school teaching. Id suggest doing a PGCE then aiming to get a job in an 11 - 18 school and moving onto a sixth form college after 2 years if thats really what you want.
However, the best advice is not to make plans before you start in teaching. When I started my training I was set on working in a private school but now I love my state school and wouldnt dream of ever working in an independent school.