I am pleased to say this is no longer a problem, and you can fly to/from anywhere in the UK (England, Scotland, Northern Ireland, Wales) including stopovers/holidays etc.
Local lockdown rules etc. may limit this in some areas.
My understanding is that the Channel Islands and Isle of Man are effectively off limits. The Isle of Man requires special permission to visit whereas the Channel Islands requires a CV-19 test on arrival (and/or you need to quarantine on arrival for 14 days).