You need the 2.1. Only under exceptional circumstances will you get anywhere with a 2.2; and if these circumstancs are exceptional enough, you uni will give you a 2.1 anyway.
Iad a friend who worked at PwC with a 2.2 in Georgraphy, graduated from Cambridge, but she is definately the exception to the rule. They have the choice of the best candidates, and if you have a 2.2 that puts in question your ability to do the training, which is definatley much harder than uni exams.
The difference between a 2.2 and a 2.1 in terms of getting a job is massive, in other words, you really want to try and get your final year down because it will make your life much easier afterwards.
If you do get a 2.2, you can still get in but some accounting experience will probably help, e.g. working elsewhere first.
I think Grant Thornton accept 2.2 for audit. Also, if you just want to become a chartered accountant, you can do this in industry and do ACCA or CIMA, these are pretty much as good as ACA in the long run.
Hope this helps, if you have any more questions please ask!