Well now that you're behaving more normally here's a few things to consider:
1) you've still got pretty got UMS, perhaps barring S1. No university would be (completely) out of reach, not even Oxford or Cambridge
2) the fact you've done BMO will mean a lot more than losing a few marks in some A-level modules. You should get an interview place, especially if you got into BMO2
3) you should learn your A-level courses thoroughly. That includes understanding how and when to round!
4) if applying to Cambridge, have a look at some STEP papers in advance of the interview. You'll be asked some problems which are harder than A-level standard. MAT and AEA are great practice too, particularly the longer MAT questions.
5) for someone who is clearly as intelligent as you, taking additional stats and mechanics modules might be a good way to go, since it widens your choice for STEP III.
I wouldn't worry about your maths UMS. I only remember my Maths and English lit ones because they're easy. It took me an hour to remember what subjects I took for A-level and GCSEs a few months ago when I was updating my CV; that's how little you'll care about them in the long run.