This thread has devolved from a specific set of facts to a discussion of permutations which number in the tens of millions.
The simple fact remains that every traveler crossing a border ought to check TIMATIC for his own specific situation, including nationality, residence, transit country, and where the passenger has been in at least the 14 days immediately preceding travel. In the case of a UK national, the specific type of passport (and thus citizenship) may matter.
For that reason, broad generalizations are relatively useless and not worth considering. All that matters is what you yourself face for the journey you contemplate. That includes testing and quarantine as well.