Wouldn't there be an element of cherry picking to this whole variant story? Like, don't viruses mutate constantly, we just don't analyze them in a disciplined manner most of the time, so if you start doing that you're bound to find something like this, but so would anyone else conducting a similar analysis anywhere else and at any other point in the pandemic?
Yes variants occur all the time but they don't tend to become dominant. This variant went from 30% of all infections in November to 60% three weeks later. In order for a given variant to supplant all the others and become dominant it must have some characteristics that make it more transmissible. That is why they are particularly worried about this variant.
London and the South East are some of the densest populated areas in the UK, and one could expect exponential growth of infections there while other less populated areas could manage to keep their Rt around or bellow zero. If this strain was more prevalent in these regions, you would also see it taking a larger share of the infections nationwide.
However, the precautionary principle has been the keystone of good handling in the pandemic, so they are right to apply precautionary measures before it's too late. We will learn more about this strain in the next few weeks.