Well I’ve found a solution that I’m fairly sure I’ll be happy with.
I went for a Dolan Dual frameset for the frame transplant. This is ostensibly a deep-drop frameset, but I will be able to use it with my short-drop Ultegra 6800 brakes with some judicious jiggery-pokery.
Firstly, the Dual’s fork is clearly a proper deep-drop fork, although the 6800 brake would probably work with some offset brake shoes such as BDOP or Aican. However, as luck would have it, I happen to have a spare fork of similar design that would substitute quite nicely. This spare fork came as part of an audax frameset I built up a few years ago, but whose fork I subsequently replaced and shelved as although it was declared to be deep-drop, it wasn’t sufficiently deep-drop to meet my requirement. Having test-fitted the 6800 brake on the spare fork, with the shoes at the bottom of their slots there are a couple of mm to spare to the top of the rim.
So I’m pleased to be able to bring a seemingly redundant hardly-used fork back into service, and it matches the Dual dimensions and colour scheme pretty well. The steerer is also the right length for the new Dual, conveniently.
Secondly, the Dual’s seatstay bridge is set at a height that would put the shoes near the top of a deep-drop brake. This means that I’ll be able to use the short-drop 6800 brake if I file the slots deeper by 2mm on the left arm and 1mm on the right arm, which would give a small amount of rim height contingency as the pads wear.
I’ve filed brake slots deeper before – on a BR-650 deep-drop rear brake that was 3-4mm too short. This worked well, and removing that much material hasn’t weakened the stirrups enough to cause any problems or concern me at all, and it’s only the rear after all. So filing 1-2mm off the slots on the Dual’s rear 6800 brake, which admittedly has a bit less excess metal below the slots than the BR-650, doesn’t concern me either, given that too is a rear.
The only thing bothering me is the amount of filing effort involved. I remember it being a lot of work with the BR-650, using a small round file, and quite awkward. Is there a better, maybe powered, tool to do this with, that would make shorter/easier work of it?
All in all it’s a good solution:
• Fast mudguardless bike back in use, but now with tyres more suitable for fast riding on Britain’s crumbling road surfaces
• Even has clearance now for 32mm GP5000 tyres should I feel so inclined
• Spare, seemingly redundant, carbon fork brought back into service
• Now have a spare proper deep-drop fork that could be used on two other (mudguard) bikes, as well as the Dual, should the need arise
• Preliminary pre-transplant measurements suggest all components including band-on front derailleur and stem length will simply transfer as-is and allow riding position to be maintained
• Even the existing seatpost is the right size
• Rear brake cable is external (old frame’s brake line/routing was internal and poorly designed, meaning it was irritatingly stiff from new regardless of all efforts to improve it)
• BSA-threaded bottom bracket
• Replacement gear hangers have also been acquired, for a bit of future-proofing as hanger designs are constantly changing
• Cost of conversion well below the budget I'd expected to be working with
Thanks for the frame suggestions - especially to Brucey for going to the trouble of investigating and photographing his suggestion.