It will help you to learn further maths, and you might find (at least at Bristol) that most students have done at least AS further maths. Don't freak out about self teaching it over this year or in summer though, a lot of concepts link with A-level maths so it won't be too hard. I've heard that a lot of first year uni courses cover a lot of what is learnt in further maths, so it will give you an advantage, but then you'll be on the same level as everyone else in your 2nd and 3rd years. If you're worried about not being able to cope, maybe you could apply to less rigorous universities? I would have a look at further maths first though, and maybe do a bit of extra reading and see how you get on.