This study indicates that CV19 was brought into UK over 1,300 times, mostly in the period 28 Feb to 29 March. Most infections came into UK from Spain, then Frances, etc. Almost no infection came in from China.
Events like the football match at Anfield (3,000 came in from Spain to watch) had negligible effect (about 20,000 people came in from Spain each day in March, aside from those football spectators).
This suggests to me that the current requirement for quarantine on arrival into the UK should have been started then, when we had little infection in the UK. I don't see the logic in having those restrictions now.