Given that you're in the UK, I'd fancy something English, as they'd be easy to find.
I don't exactly know when Raleigh-Nottingham started/stopped making cassette-geared bikes. I've seen some early 70's stuff that'd probably work. I've been watching eBay for something to use as a project and find out what exactly will work for derailluers and cassette. I've been messing with early 60's stuff. I currently have a '63 BSA, '63 Phillips Ranger, and '64 Armstrong. I, also, have a couple '79 10-speed models, but, they're made in Thailand and not near the quality of the Nottingham stuff (too nice to see go to the scrapyard). Anyway, my 60's bikes are all 26", 3-speed Raleigh Industries (RI) bikes. Everything I've seen, except for race bikes, have dropout eyelets and are lugged (not high-end cast). They easily can be upgraded with modern components. The 3-speed frames have 120mm rear and 90mm front spacing (my Thai 10-speed bikes are 130/100, respectively) and no derailluer provision, so 3-speed frames are out. The BB and headset are 26TPI, but, that's no problem. The steerer can be re-threaded to 24TPI by any bike shop. That'll allow you to run any modern 1"X24TPI headset. The BB is the same diameter as Campy and can be re-threaded, too. That'll allow you to run any Campy-dimensioned BB. Modern 700c wheelsets fit, using standard dual-pivot brakes. A 25mm tire is all that will fit with 35mm fenders. Otherwise, you can run up to 32mm, depending on actual dimension. The seatpost is 25.4mm and the stem is, also, 25.4mm. That means you can run a modern seatpost and quill or adapter stem. So, with all that, I'd try to find an early 70's, Nottingham-built, 26", 10 speed Raleigh Industries bike. They ride super-nice and are compliant.
To get ideas started, this is my BSA single-speed project. The bar and seat position are just slipped together. It's actually on the road, but, awaiting decals. Maybe, I'll update this thread, when it's done.