He's a former spy who was jailed in Russia and then handed over to the UK in a prisoner swap deal. In terms of intelligence information he's a busted flush, of no further threat to Russia, so so I don't see what they have to gain by assassinating him. On the other hand Russia's enemies may have a lot to gain with a false flag operation on British soil.

For the record I am neither pro or anti Russia, i'd just rather operate on the truth.