What sort of grades to Magic Circle firms look for?

You have a chance at MC firms, but would be a below-average applicant so would need to do plenty of well researched applications, do what you can to improve your desirability as an applicant and hope for the best.

You certainly have a great chance of picking up a decent TC in London

Out of curiosity, what puts him below average? I would have thought a 2:i from St Andrews was probably on par with a 2:i from the redbricks, and AAAABB at higher is pretty strong. Is the average calibre for MC more like redbrick first/Oxbridge 2:i or better?

There are many people with worse academics than the Op, but I think the average calibre is probably more like redbrick first/Oxbridge 2:i. There are an awful lot of Oxbridge grads in MC firms, I'd guess about 40% of my intake. No reason he can't get a TC on those academics, but the market has become very competitive.