See here from BuzzFeed:

Now, BuzzFeed News can reveal that Puncher is among at least 14 people US intelligence officials suspect were killed in the UK by Russian mafia groups or secret services, two forces that sometimes work together, since Putin’s rise to power. Four American intelligence officials said US spies have gathered intelligence about the scientist’s death and believe that he “was assassinated”. They said they have passed MI6, Britain’s secret intelligence service, information connecting Puncher’s death – and 13 others – to Russia. Yet the British police have ruled out suspicions in all those cases and shut down any further investigation.
How can MI6 recruit agents in Russia when MI5 cannot protect defectors who become British citizens? Unfortunately, British intelligence probably has a similar reputation with disgruntled Russians as it did with disgruntled Soviets from the late 1940s to early 1960s.

Of course, having uncovered the most nefarious Russian intelligence operation since the penetration of the Manhattan Project, spymaster Christopher Steele is the jewel in Britain's crown, and his extensive training has enabled him, his family, and his pet(s) to escape the SVR and GRU...