Jaboet
If say that you have a SA ATP and fly the 737, then you can just buy the set of manuals study them ( e-bay ) , come over to the UK and sit them. Because of your experience you will not have to be enrolled to a groundshool course and do the 4 weeks of class room study on a distance learning course.
Sorry but I cannot answer your question about the medical, e-mail the UK caa directly and they will tell you.
GGW