Mudguards

The difference in the radius between a 700c rim and a 27" rim is about 4mm. If that's going to be an issue then you don't have enough mudguard clearance!

P.S. I'm using Giles Berthoud 40mm mudguards on my 27"-wheeled Galaxy, but then I'm running it with 700c wheels and 28mm tyres.
Clearance 'twixt wheel and frame/forks isn't a problem: there's an echo.

I was more concerned with wheel/tyre, but Adroit explained that for me.

GB would be nice, but beyond my means and ambition for this build: mine is "Aldi" to your "Marks & Spencers".