When I looked in to joining the Royal Navy, they required visual acuity of 6/60 uncorrected and a maximum correction of 5 or 6 dioptres. That was for entry as an Engineering Officer. The requirements for Aircrew Officer were tighter still (2 or 3 dioptres maximum correction, I think). I didn't look at the requirements for ratings. I think the requirements for the RAF were similar.
Note that this was three or four years ago, and I can't remember all the details.
If you're interested in joining, call or visit a Careers Office and ask!