Tatt, I would have thought doing early ASs would allow you to keep more career choices open rather than the other way around.
What subjects do you think wouldn't work KT? I can't think of any.
A grammar school where a friend's ds goes offers ASs in English Language, General Studies, Performing Arts and Science for Public Understanding in year 11. They all do English GCSE early then I think either retake or do the AS and the science GCSE modules are sat early too, again some retake & others do the Science AS or focus on other GCSEs.
I don't really know how that sits with sixth form studies - I imagine if you are science-oriented you would really want to do some combination of Bio/Chem/Physics rather than take Science for Public Understanding further and English students might be steered towards English Literature rather than Language, so maybe they tend to start a new set of ASs in L6 and do sixth form in the usual way but end up with a wider range of AS levels than usual.
My dcs school plan to offer Computing A2 as a yr12 option (as a 4th or even 5th AS choice), but the disadvantage is if you decide you don't like one of your other AS choices you can't drop it for A2 because you can't continue further with the computing if you see what I mean (ie if you did maths, physics, french AS and computing A2 in yr12, planning to do maths, physics & french A2 in yr13, then decided you really, really didn't like physics, you wouldn't have another subject to do in its place at A2 level). The other option if you have done early ASs is to do different subjects for a year and then resume some old ones in U6 - giving you more choice of A2 (and maybe careers) at around the same time as you are choosing university courses - so that keeps your options open for longer really. Ds is hoping to do Chemistry AS early, do a different AS in its place in yr12 and probably then pick it up again in yr13 for example.