UMS = Unit maximum score (or something like that!!)
Every single A-level is 240 UMS marks = an A
That's the point of UMS - they standardise marks. That's why if you take a paper with 120 marks and a paper with 90 marks then they are both marked out of 100 ums. (That's why all A-levels are out of 600) It standardises everything, so e.g. for a really hard English paper 92-100/100 = 100 UMS, but then for a maths paper it's more like 59-60/60 = 100 UMS.
You need to be looking at about 80%... it's a bit different between subjects, but 80% is the UMS u need for an A.