"It is already in Country X"
A number of commentators say that the flight bans are pointless because this observed "new" virus strain has already been found in a number of other countries.
Travel bans can still make a lot of sense, for as long as you have reason to believe that a plane load of travellers from the UK could introduce a number of new spreaders and therefore speed up the spreading of the new variant at even faster pace.
Fewer spreaders does buy you (some) time as a government, even if the variant is already in circulation.
Clearly there is also a useful element to this in terms of warning people to simply not travel, full stop, as the situation remains unpredictable.
And yes, who knows, also provides a nice test-run/cover-up for End of Transition period related issues on both sides, but wouldn't believe that this was the key driver here.