Since September 11th the US and UK have showed they're not even willing and able to organise the covert manufacture of nerve agents to "discover" in Iraq to avoid many leading political figures being permanently discredited when nothing was found, never mind manufacturing and using nerve agents to bump off a couple of Russians in a public place in Britain for no obvious political benefit.
If you genuinely think the UK needed a couple more dead Russian dissidents (and pretty unsympathetic ones at that) to justify stricter sanctions on Russia, you really, really don't understand the relevant politics.