Theres no point worrying about it until it happens. I thought I'd failed one of my exams in january cos i dwelt too much on the bits i messed up instead of thinking that i'd done alright on some other questions.
If you get BBB, theres a good chance they'd still accept you. (My A level grades were BBB, which also happened to be my offer, when I started at manchester in 2002). You'l be able to ring them up and probably get a fairly quick answer as to whether or not they'd accept you.