Checking the GCE AS and A Level Subject Content for Mathematics we see the following on the use of technology:
Will Hornby, OCR Mathematics Subject Advisor, has written very clearly on calculators for Mathematics A Levels in this post.
For excellent advice on the use of Technology, see these MEI pages. Note MEI’s comprehensive guidance with ideas for integrating Technology which I think will be useful whichever examination board you are using.
You can find an excellent calculator guide for the Casio FX991EX-ClassWiz on Dr Frost’s site.