You will earn more than enough in medicine to be comfortable, except perhaps in London you may be stretched a little thinner, even with extra London pay.
Mathematics at university is entirely unlike at A-level, really. I'd suggest taking a look at Spivak's Calculus to get an idea of what it entails. Broadly, at university in mathematics you will be concerned with finding general solutions to problems, rather than specific ones as in A-level, and then proving rigorously that those solutions are "correct". That's at least the most laymans way to put it I can think of...