...don't worry about your Uni mark, to me it sounds like you've done great and you've got yourself a good job!
I am not sure how much 2/3 years in audit and an ACA would put you in a good position with strat firms. You wouldn't be joining as a fresh grad and they'd want to see some transferable skills (project management, client facing roles, strategy related teamwork, etc) which I don't know how much you would gain in audit.
There is the usual numbers issue. PwC recruits hundreds of audit grads nationwide and a handful of people in its strategy division in London. It's a similar story elsewhere.
I would see 3 possible routes you could take:
1) work hard in audit and ask for an internal transfer to something more related to strategy, like TS, and then move on after a year or two.
2) I'm assuming you're probably bilingual. It's a long shot but they may require your language skills on a strategy project...Another point about language skills, I think that in some countries they would probably be more inclined to take you into strategy with audit experience from PwC if you fancy working abroad
3) Do your ACA, work in industry for 1/2 years, and then do a good MBA
However, I wouldn't drop the PwC offer. It will definitely be a great experience and you've done very well to secure it.
Hope that's of some help!