You might be thinking that teaching A-level is more satisfying/less stressful/easier/more intellectually stimulating than teaching in schools because unlike school kids, 'they want to be there'.
Many modern VIth form students are eye-poppingly lazy, have no independent study skills whatsoever, are startlingly bad at basic grammar and maths, have very poor attention spans, expect to be entertained all the time, expect everything to be 'easy' (so give up immediately if the answer is not obvious) and have a hilarious lack of basic general knowledge. They want A-levels, and to lounge around drinking coffee/falling in love as if they were extras in an episode of Hollyoaks, but they don't actually want to do the work. Many have shuffled into college because of parental expectation, or the fact there's really not much else they can do. Teaching them requires skill, passion, energy and relentless nagging. And post-16, the funding of the college is determined by 'success rate', so any student who fails or drops out is a Problem... often more so for the staff than the student. You will be held responsible by everyone for your students' results, irrespective of whether the student has ever completed their homework or turned up on time. Don't you know that A-levels are so easy now, everyone gets an A?
In the light of this, the best way to get the skills you need to teach A-level is to teach secondary first. At the very least do a PGCE and spend some time in schools. Teaching is many things but on one level it is a craft and there are skills that can only be learned the hard way I'm afraid.
Least you think I am bitter and twisted, A-level teaching is a GREAT job, but a lot of people have a very idealised view of what it entails.