What Boris basically means and what the scientists are worried about is:
Not the Kent or original wild strain of the virus reinfecting us from Europe, we already have it within our population. That strain will not be going away until we achieve around 80% immunity.
They are concerned about the South African variant, this variant is better able to avoid the immune response. Its basically rendered the vaccines much less effective, although they are not completely sure by how much. Early evidence shows the vaccines are not very effective for the SA variant.
So Boris, rightly so bans holidays and adds several EU countries to the red list, good move. As the SA strain is now grounding itself within the EU. If it finds its way into the UK and manages to circulate, we are screwed.
We have more vaccines being made right now, for the SA strain and our scientists are referring to them as boosters.
The problem, is that we may not be able to keep up with mutations and as the virus is now being put under pressure, by growing immunity. It will evolve through natural selection. The EU isn't so much our worry, it's Brasil and India and any country with poor health systems which aren't locking down. They are breading grounds for more aggressive mutations, which will eventually find there way here.
Our only real option is to lock our borders.