British governments like to justify refusals to follow rational policies, or reversals in the face of disasters (following a refusal to follow rational policies) with spurious scientific claims of a change in circumstances. Claiming a new mutant covid enables them to tighten up restrictions while also pretending to previously have been right about loosening up restrictions.
It's similar to how they can't have a rational cannabis policy because of a nonexistent varient called "skunk" which is so much worse than the cannabis that nearly everyone has experience with that it has to be regulated exactly like all cannabis always has been (even back when they weren't making claims about "skunk".) This "skunk" conveniently actually has all the traits that they have always claimed that cannabis had.
I believe in this case your reasoning is flawed, because all countries will be quick to shut down flights with the UK and no politician in the UK wants that.
It's similar to how they can't have a rational cannabis policy because of a nonexistent varient called "skunk" which is so much worse than the cannabis that nearly everyone has experience with that it has to be regulated exactly like all cannabis always has been (even back when they weren't making claims about "skunk".) This "skunk" conveniently actually has all the traits that they have always claimed that cannabis had.