Many (or even most) of those people are in London because they either don't like the Putin government - or don't want to live in Russia because it's not a great place to live. so exactly what good would targetting them do? They're not Putin's supporters, so why would he care if we sling them out?
Plus you sort of need reasons to confiscate peoples' property or throw them out of the country. That's why our system is better than the current one in Russia
After all, this is one of the reasons why Putin behaves this way. He can no longer tell people that his lot are in power because they're brilliant at stabilising the economy after the chaos of the Yeltsin years, because the economy's been doing badly and corruption has ramped up to similarly extraordinary levels. So now he has to wrap himself in the flag instead.
Many of Putin's closes allies are under travel bans, so they aren't in Kensington. If they were, they wouldn't be running Russia, and we wouldn't have so much of a problem with them.
Now Putin's own regime and his own supporters also keep their money abroad. Because they also don't trust the system. So they would be a legitimate and sensible target. If we can track their assets to London, then we can seize them. London and New York are the financial capitals of the world, so some of that money is likely to be there. But can we prove it? Lots of Russian money could be elsewhere, such as Cyprus or in fact anywhere else. If I were close to Putin my money would be hard to access for the US and UK government, given that they've been under some financial sanctions since the Crimea annexation. And many Russian companies have struggled to borrow in the West in the last couple of years, since people were nervous there would be even harsher sanctions in future.
An aggressive use of things like unexplained wealth orders might have some effect though.
With global cooperation we could do more here. But that's hard. Many EU countries want to reduce the post-Crimea sanctions, not increase them. And there was no support for the UK after the Litvinenko murder, so I'm not sure if there'll be much more now.