Ok, fair enough but how do you do that, how do you police it?
It's easy to identify a flight that arrives UK from China non-stop, or a flight, if there are any these days, that's a direct (i.e. from China, no aircraft change but a transit stop enroute).
OTOH it gets more difficult with identifying individual passengers who have maybe flown with a Chinese airline to, for example Seoul and then connected so arrives in London on KAL, or connects via Singapore and arrives with SIA or flies into Rome with a Chinese airline and then connects into London with for example Alitalia or BA. Umpteen other options are available...
Even if you do "weed" those bespoke individuals out at say Heathrow immigration they've already been sat next to other arriving passengers at the intermediate airport and on the final flight......
As I see it if you want a watertight quarantine you need to get China to restrict or even ban all those departing China ...or you have to put in quarantine for all UK arrivals, regardless of their provenance...anything else is window dressing (not that that has stopped politicians before...)...
And we've been down that road before..............................