I was going to write 32mm based on GP5000s but then thought better not go too far, people will get upset.
The last tyre I bought was 2.35" Mezcal and I'm still using up my road tyre stash from the last 10 years so have plenty of 23, 25, 28 to get through still.
Does the UK even have any roads that aren't basically gravel now anyway?
Everyone telling themselves they're running 28mm tyres because "aero" but it's really because the roads are too fucking shit to run 23s any more.