Is there any way that B could be moved up to an A before you apply? I don’t just mean on paper, I mean could you work that bit smarter/harder in that subject to get the A? Would a personal tutor be out of the question? You can find them on FB or by teacher recommendation. My friend got one in year 13 and their English grade went from a high C/low B to an A in the actual exams. Made a huge difference, but they did put a lot of work in and had extra ‘homework’ from the tutor to complete too.
Personally, if I were applying I’d maybe only go for one at A*AA if I was predicted A*AB as law is a super competitive subject and any decent ranked uni (especially those wanting LNAT) will get many applicants per place. Not being predicted the minimum entrance grades, unless there’s exceptional circs or contextual, is an easy way for them to sift you out for a rejection (sorry).
On the flip, if you do better in the exams you could always take a year out and reapply with grades in hand.
Good luck.