. Maths and a science is very important and I think should be compulsory.
I'd disagree. I was pretty good at Science GCSE, I got a double B grade without putting too much effort in, the step up to chemistry and physics a-levels was immense. I couldn't cope with it and consequently dropped physics after AS.
There's no point doing a subject you're going to get a poor grade in, because unfortunately more often than not it works on UCAS points NOT the a-levels you took.
You get some dappy git who gets three A*'s in drama, RE and media and you get a person who gets three B's in English Literature, Biology and Maths, it will be the person with the most UCAS points they are more likely give the place to. It's unfortunate and a sad fact of life that they can and do sometimes reduce the numbers just by looking at those of whom did not get the required grades.
Even now, applying for training contract application for law firms - having completed my a-levels, a degree and now doing a post grad law course - are all asking on the application forms "How many UCAS points did you get?" and then want you to list the subjects and grades.
All the waffle about why you're "the better candidate" more often than not is just used to decide between candidates with very similar grades.
So my advice, stick to what your good at. If it lends itself heavily to the course you want to study at university then all the better. i.e. you want to study English at uni, study English a-level, you want to study history, study history a-level, you want to study Law, don't study law a-level (they don't like it in law apparently, I assume because the law changes too much between the years and they don't want all these cocky kids coming in thinking they know it all already)