Originally Posted by KARFA

So if you are doing a same day transit that would be fine anyway, which would cover you for your MEX-LHR-Switzerland when you pass through LHR on 6 September.

In terms of a later flight back to the UK, if you were last in a red list country on 5 September as you note then if you came to the UK on 15 September or earlier you would be a red list arrival. You need to arrive no earlier that 16 September to avoid that, so yes I agree with your reasoning as you would have a clear 10 day period before (6 to 15 September) where you had not been in or transited a red list country.

Arriving in the UK on 16 September should be fine and you won't be a red list arrival.