I did in the end manage to get hold of a natural titanium iPhone 15 pro from Apple in Norwich.
Did the trade in in-store and Apple gave me £150 for my old iPhone 11, which considering was 4 years old, had 80% battery health and a dodgy charging port, I didn't feel robbed.
The iPhone 11 was my first iPhone so I've never done a phone transfer before - but I was amazed at just how well it works - I backed up my iPhone 11 just before I left, factory wiped in store, and when I set up my new phone at home, it was like the same phone - everything came over and just worked, with the exception of my Apple Watch, which needed to be wiped, and re-paired (everything gets restored exactly as before).
So a good experience. Happy with the iPhone 15, glad I didn't got for the Pro Max, it's a perfect size, imo. Not cheap, but feels like a massive upgrade from the 11 - the screen, and although I've got a "proper" camera, the camera system is impressive. I'll try and get around to posting some comparison shots.