The most important A-level is probably Chemistry, although some Uni's accept Biology instead. If you're taking 3 A-levels, the best combination of subjects might well be Chemistry, Biology and either Maths or Physics. The minimum grades to get a place at a dental school are usually ABB. Work experience, either paid or unpaid, assisting in a dental surgery is likely to boost your chances of getting a place.
There are dental schools in Belfast, Birmingham, Bristol, Cardiff, Dundee, Glasgow, Leeds, Liverpool, London, Manchester, Newcastle and Sheffield. When I was teaching, the advice I always gave to A-level students, seeking a university place, was to only consider universities which were at least 100 miles from their parents' home. (You'll won't enjoy university life to the full if you 'pop home' every weekend or, worse still, if you live at your parents' home). Then choose a city which has a large student population. (Almost any city I've listed above meets that requirement).
Further information here:
http://www.bda.org/education/docs/careers_in_d entistry.pdf
Chris