with a maths with mechanics qualification, am i bale to study math at uni?

Question

or does one always have to study double maths for university?

Answer 1

You would have to check the prospectus of the universities you want to apply to. However I would be very surprised if doing double maths was a requirement. I think you will find you need a good result in maths plus any other 2 subjects.

Answer 2

As above - you should be fine but contact the universities that you are thinking of applying to first. Most (decent) universities will ask for at least a grade B at A level plus another 2 Bs.
The first semester usually covers the further maths content anyway.