"No single intervention, other than a complete, pre-emptive closure of borders, or the
mandatory quarantine of all visitors upon arrival in designated facilities irrespective of testing history can fully prevent the importation of cases or new variants" https://www.gov.uk/government/publications/dft-and-fcdo-international-importation-border-and-travel-measures-21-january-2021

*irrespective of testing history*

So why bother with testing of those that you let in? SAGE told them it's a risk.

"Evidence from the continued spread of the South African and UK variants suggests
that reactive, geographically targeted travel bans cannot be relied upon to stop
importation of new variants once identified, due to the time lag between the

emergence and identification of variants of concern, and the potential for indirect
travel via a third country (moderate confidence, moderate evidence)."

"WHO analysis suggests that partial travel bans only have the potential to delay
spread and peak by a few weeks or months. Extensive travel restrictions (over 90%
of journeys stopped) would be required to meaningfully affect the magnitude of an
epidemic."

You know what usually persuades 90% people their UK trip wasn't essential afterall? Having to spend 10 days in the covid-19 quarantine hotel! It's a effective in itself and acts as a fantastic detergent.

"Geoghegan, J L, et al (2020) finds the introduction of a New Zealand lockdown in
March 2020 resulted in fewer than 20% of introductions into NZ leading to more than one additional case"

Also they know where the infections came from! Keep numbers small and you can trace most.