Asset confiscation, Comrade Stalin would be proud. The reason such things are not done, at least not without a lot of due process and evidence for why it should happen, is that you then destroy the confidence of anyone to own UK based assets. If the government can simply seize them because they don't like you or the country you're from. It's the kind of thing tin pot dictators (and communists!) do and then a large part of the reason why their countries usually turn in to holes where no one wants to live.... Just look at Cuba and Venezuela!
Abramovich owning us is no worse people connected to W Bush owning clubs 20 years ago when the US invaded Iraq. Abramovich is not directly involved in Russian politics, Putin or the Kremlin, he just happens to be Russian. I guess the logic is the people in western countries are better than everyone else, so when they start wars of aggression based on flimsy evidence which subsequently is shown to be totally false, "it doesn't count" and they don't face any consequences.
If there is some evidence Abramovich is responsible for what is happening in Ukraine or that he has any influence over Putin and what he is doing, let it be presented and then it judged on it's merits or not.