I didn’t have to get a letter from my doctor, MYA just asked for my permission for my medical records to check I had no problems that could affect my operation, which I gave and all I had to do was pay £20 for them to be sent over.
The main reason they ask for your medical records is to avoid any complications in surgery because some people fib about their history! Also if you don’t want the doctors at your clinic to know then you can always join another one; you’re allowed to be registered with 2 I think.
This really wasn’t a big issue for me and I didn’t have any problems, but I guess everyone’s different!
I hope this helps a little xx