Having worked in one compartment does not give you full access to all compartments, especially that dealing with state-sanctioned assassinations. People are regularly assassinated by US and UK forces, whether it's the SEALs or SAS or some other group. Not sure why it would have to be an intelligence officer carrying it out. Nor do I believe, if they were, that all intelligence officers would be let in on it. Porton Down poisoned thousands, some willingly, some unwillingly.
I think it's 160 countries who haven't expelled Russian diplomats, if the figure is accurate. And among those who have, we've heard from the Czech president that despite expelling some diplomats to show solidarity with the UK, he has not been presented with any evidence and he wants to see it. As for the US State Department, when asked if they have seen evidence, their position was "I haven't personally seen evidence but why should we doubt Britain's word." This is rather different from saying "we have. I haven't personally but I'm told we have."
Let's remember as well that when we throw around names like "UK, Russia, US" etc as I do myself, it often just means the leadership not what the rest of the world is actually thinking. There is a lot of scepticism about this within Britain itself, that is not proportionally reflected by those in Westminster who are less free to say what they believe, less free to ask questions, and have been shown the same evidence everyone else has, which is zero.