Putin’s election win is not in doubt but it’s not about whether he wins or not.
Apparently the Russian government work on a 70-70 model for elections. The theory goes that Putin needs a 70%+ turnout and of those voters he needs 70%+ to vote for him. Any less and it encourages his opponents out of the woodwork.
From what I have been reading there is a real danger of voter apathy in these elections meaning you might not see the 70% threshold reached.
This assassination has come at a great time for Putin. Nothing like a bit of nationalism to drum up a vote before an election, a bit like Thatcher invading the Falklands, and if it causes further chaos in the West, all the better.
He would have to be a pretty major suspect at this point. He certainly has motive and opportunity, and that is without seeing any of the evidence the UK government is privy to.