It would help with A-level maths quite a lot - provided you actually do well in it. I took further maths as a 4th and I barely needed to revise A-level maths at all during year 13. If you're interested in maths as a subject and you enjoy (or at least, don't mind) doing the work then go for it.
How much it'll help for uni applications there depends on what course you have in mind. If you wanted to apply for maths then it's obvious, but for NatSci at Cam or physics or chemistry at Oxford, somewhat less so.