Everyone's input has been really helpful. Thanks for all the advice and explanations of gear. I think I'm at a point where I need to visit the frame builders and work it out. But, on paper, I'm thinking:
Frame: BJ Audax End-End, A Woodrup (if they've got something ready made that fits out the back), possibly a Spa Audax (?). Mercian is a bit further away and I think will be too pricey, same with Ellis Briggs.
Wheels: Either Open pro or H Plus Son rims. No idea about the hubs. Any suggestions? Still love the look of the Pacenti Brevits too.
Groupset: Spa TD2 chainset - undecided on triple or double at the mo. I do NOT like hills, but I love coming down them.
Still don't know about mechs. I liked the idea of campag, but I have no experience with them. Do they even work with a triple chainset (excuse my ignorance)? I also can't use campag brakes, because I'm using mudguards. It's possibly looking like 105, but that feels so dull. Centaur Levers (but obviously depends on mechs) - I have no idea about compatibility. Cassette - ?
Brakes: Shimano R650. I thought about Tektro 737 just as an alternative to Shimano, but have read that the 650s really are better.
Handlebar and stem: Nitto probably.
Pedals - Shimano m540 or a520
Tyres: Gatorskin folding
Mudguards: SKS chromoplastic
Saddle: Brooks B17
Feel free to suggest alternatives. I've just been enjoying building this in my mind, like a dream team, but I'm not married to any of it. That said, a lot of my choices are based on the component being silver. Prettiness is a big factor, along with comfort and durability.