The medical renewal is no different from your initial medical examination, regardless of where you take it.
The exception being of the requirement for an ECG and Audiogramme, which only have to be taken periodcally, not every year - dependent upon your age, but if you haven't had a UK CAA examination since 1986 you'll definately require the full medical.
What you may find is that the content of the examination has changed, i.e blood/colesterol checks.
There's still not total parity between the UK CAA and JAA medical class one and eyesight was one of the issues, the JAA were stricter than the CAA.