I can't agree with those who say "don't bother". If you can afford one, why would you choose to take an exam which allows one without it?
To me, that's like going to a tennis match but leaving your shoes at home!
If I was taking A level now (instead of teaching it), I'd take a bank of calculators in with me. Certainly the Casio 991EX (latest update of the 991ES) and the Casio GC20 graphical. If I could be bothered to learn how to use the Ti-Nspire CX, I'd have one of those too.
Why not take everything in? It might not make any difference but when 1 mark might mean the difference between a place at your first choice Uni or having to find something else in Clearing, they why not do this?
And if you can't afford it, ask at your school. Many schools have parent associations that support students with things like this, buying some for the maths department that can be lent to you for the course.