The UK isn't a major superpower though. Russia sees the UK as the poodle of the United States, and although it loves it's poodle, poodles can easily be replaced. Putin was sending a very specific and threatening message to any Russian dissidents living in the UK. You aren't safe even if you thought you were. Shut up.

Hardly swept under the carpet. It's an ongoing diplomatic headache. Just a couple of weeks ago, a public enquiry (which Litvinenko's widow had pushed for) publicly announced that Litvinenko was most probably murdered by the FSB with Putin's approval.