I suspect there is still a lot of Russian money in London. London’s dominance as a world financial market makes it advantageous for countries all over the world to invest some of their money there, even if there is the potential for the cash to be frozen.
Money means zilch to egomaniacs like Putin.
That would be like some court somewhere producing a judgment against Obama, and enforcing it by seizing ExxonMobil's operations in the country. Yes, they could, but it would be very banana-republic-ish.