It depends on your percentages...
For the average exam, 80%+ constitutes an A grade, 70-80% a B, and 60-70% a C etc.
So, add your AS and A2 percentages together and divide by two. If you got a low C (60%) in AS and a low B(70%) in A2, you'll get a mid C overall (65%).
A2 exams are more difficult than AS, not easier... the only reason that people do improve is because they buck their ideas up.
I think this is correct, I don't really understand what UMS means sadly.