It may be worthwhile contacting the UK CAA medical branch. When I converted to a UK ATPL in 1999, I was able to sidestep the whole Class 1 initial issue business. I had to have a recent chest xray, opthalmologist report, current Class 1 medical (from Australia, where I was coming from), audiometry/hearing report, and that was it (as best I remember). My GP was able to authorise all of those as a referral, so Medicare covered it.
Saved me a ton of money, time & aggravation. Maybe that's still possible?