Firstly, the majority of people I know of in year 12/13 did very badly in their January exams. Not sure whether it's a coincidence or something's going on with the exam boards, but I don't recall results that bad when I was in that position.
Secondly, AS grades aren't neccessarily a deciding factor. I got BBCD in my AS levels and picked it all up and the course I'm on now is AAB requirement. Obviously we're not talking Us there, but the general point is that it can be turned around.
In terms of actually failing and having to re-sit - I know someone who failed his AS year, and re-sat. At the end of the 3 years he had AAB at A level and is studying at Newcastle uni. If that does happen, you just need to be clever and work it in to your personal statement. Say things like 'I took the difficult decision to re-start my A levels and succeed in what I discovered were the right subject choices for me' or something along those lines. That's what he did anyways. Your chances aren't ruined. Good luck.