There were no Grandfather rights when the CAA Class One became the JAA Class 3, when it came to renewal time the former became the latter, simple as that.
I may be wrong, but I was under the impression that the RAF ATC Medical is exactly the same as the old CAA Class One, indeed many Mil' ATCOs had their medicals with local AMEs - you may be able to do the same.
The eyesight requirements for the JAA Class 3 have often been confused as being the same as those for JAA aircrew medicals, I am informed that the ATC requirements are lower.
As has been said though, Gatwick or your local AME will advise.