Non UK>UK transit > non UK given how things have worked so far I think you are correct.
The rules keeping evolving but
AFAIK there’s never been a requirement to stay in the UK whilst a testing or self-isolation period times out … there’s usually been a clause that allows you to leave isolation to travel direct to port of departure to leave the UK.
With regard to the question about arriving at LHR, overnight there, then onwards next day to e.g. MAN or NCL it might be worth being aware that as a point of interest the rules for non-vaccinated arrivals contains the clause:
”Long journeys to your quarantine accommodationIf you have a long journey within the UK to arrive at the place where you’ll be quarantining, you may be able to stop overnight in accommodation where you can quarantine yourself from others before continuing your journey.”