@JonBoy Similar-ish situation. I'd been using a Japanese ATPL and Class 1 but wanted to renew my CAA type rating and, therefore, my CAA ATPL. I think my last CAA Class 1 ran out in 2009.
Of course, it was far too much to expect the CAA to accept another ICAO medical. The Japanese are, after all, known for their really low medical standards
(I have no idea how I got through each year; they are MUCH more difficult than the UK). The Ministry advised I would have to undergo an Initial Class 1.
Take comfort, if this overweight, bit of hearing loss in the port ear and 'that eyesight chart looks a bit blurry now' 55 year old can pass it then there is hope
The AME at Heathrow Medical Services did write something like 'Repeat Initial Class 1' on the top of the form and explained this differentiated it from a 'real' initial medical. To be honest, it was just like a regular Class 1. I did have to do the peripheral vision test, which I don't recall doing on UK renewals but the Japanese bring that test to a different level, so it was a relief to just have normal blobs of white, instead of teeny, weeny ones as you look through a lens
It cost me £570 to bribe the doc to sign the bit of paper and all went okay after that. The people at LHR were very efficient and the doc very pleasant indeed (not just because he passed me either).
Believe it or not, I've found it quite refreshing being back in the UK system. At least you can speak to the doc and not everything is a huge secret, with things written on your notes with no discussion about them.