Depends a bit of whether you are sticking with the traditional form factor (ISO) or decide to go all in with one of the many QMK-based ergo designs.
Although I didn't original plan to, I have ended up switching to ergo so I'm not sure on the state of programmable ISO boards. I suspect the majority of fully programmable traditional boards will be ANSI.
Pre-made split ergo keyboards examples (not cheap though)
- Ergo Dox
- Keyboardio Atreus (I have one of these)
For UK based, I just noticed https://mechboards.co.uk/ now have assembly service for various keyboard kits. Haven't used them myself though.
There's also Falba Tech in Poland who will assemble and post to UK
Probably others too.
Sadly being able to get fully programmable mech boards from mainstream retailers is still a way off.