Seems odd your school would only predict BBB based on your GCSEs, given you aren't taking any science subjects now. However this may be a cause for concern for the university - the GCSEs themselves aren't of great interest, but a low prediction at A-level, unless explained in your reference that these are somehow generated arbitrarily and your referee can convince them that you are going to get A*AA+ , you may well have difficulties getting an interview invite (even excelling in any pre-interview assessment alone may not suffice).
If you are fixed on Cambridge Law, you might want to consider the possibility of taking a gap year and reapplying once you have your A-level grades (at, or ideally above, A*AA). However if you do get good reference as above and good performance in any relevant pre-interview assessments may get you an interview, after which it's all in the air. Just be realistically about the possibility of it not getting to that stag - I would recommend doubling down on your academic work and trying to do as well as possible in those (especially History).