Unfortunately the news report I've been looking for on our local rag's website seems to have disappeared. However you can get the gist of the story from this slightly earlier report:
https://www.eadt.co.uk/news/stena-quarantine-confusion-1-6792502
As you'll see from that report, the ferry from the Hook of Holland to Harwich was due to enter UK waters at around 3am but not dock until 6.30am and, as quarantine regulations were due to come into force at 4am, there was a question mark over whether the passengers on board would be obliged to self-isolate or not.
A later report though (which is the one I can't now find) said that the relevant Government agency had confirmed that it was the
time of docking that counted and NOT the time that the ship entered UK waters. (Stena Line then said that rather than cruising slowly overnight, as the ship usually does so that passengers can get a good night's sleep and arrive at a sensible time to start a new day, the vessel would travel at maximum speed throughout its journey in an attempt to dock before 4am).
It seems logical that a similar argument would apply to flights. i.e. it's the time of landing that counts, rather than the time of entry into UK airspace.