Toodle off to
Medical Examiners Database Search | Medical | Safety Regulation and find the best one for your location.
Book yourself in, up to a month before I think. There is a form you need to fill out and take with you on the CAA website too...
Documents for Download | Medical | Safety Regulation look for the Med160 ones under JAA. One is the form, the other the guidance notes on what to put where.
Don't forget your old cert, logbook, forma as above and cheque book.
{Third edit to correct typo, I'm sure my eyesight was OK earlier in the week at my renewal!}