Why Are We Quarantining the UK?

Grant Falvey/London News Pictures via ZUMA

Let our journalists help you make sense of the noise: Subscribe to the Mother Jones Daily newsletter and get a recap of news that matters.A new, more infectious variant of the COVID-19 virus is wreaking havoc in England, so now everyone is panicking and cutting off travel and commerce with the UK. But wait:
If a new virus sounds scary, a new mutating virus sounds scarier still. In Kent in September, scientists now believe, somebody with Covid was the unlucky first person to pass on a variant form of the coronavirus that is maybe as much as 70% more transmissible than the version we have been used to.
September? That was three months ago. So what are the odds that the B117 variant is still confined to England and hasn’t already taken up residence in all of Europe? Or even all of the world, for that matter? IANAE, but it sure seems like about zero to me. Here’s a report from BioSpectrum:
The new virus strains spread 70% faster with similar lethal effect; Cases reported in South Africa, Australia, Italy, Spain and parts of Europe.
The Washington Post reports the same:
Australia, Denmark and the Netherlands have identified cases of the variant in their countries…On Sunday, Italian officials announced that a patient returning from Britain “in the last few days” was in isolation after scientists detected the mutation. In South Africa, health officials said Sunday that a version of the virus similar to the U.K. variant has been found in 80 to 90 percent of samples analyzed since mid-November.
Is there really any point to quarantining the UK? What am I missing here?