“Of course Vladimir Putin, the Russian executive power, is responsible for foreign relations including extraditions. And the Briths expulsion of the four 'diplomats' (the tacit understanding is that they are SVB/formerly KGB) is a response to the refusal to extradite Lugovoi, not in response to the murder itself”
So, if I follow you, France should have expelled few US and English diplomats when these two countries refused to expel persons involved in the terrorist bombing campaign…
It is a new kind of justice:
Somebody from your country killed somebody in my country and fled back in your country. Because you refuse to extradite him/her, I expel your diplomats, knowing you will do the same.
Will be exclusivity for Russia or will it be extended to others countries, and others crimes… No, because we have few cases with Canada ….

“You refuse to take sides”: I do take side, I do. Just as much as I refuse political murder (still to be proved, because I still don’t think Litvineneko was a danger for Putin political carrier which will be ended next year if Russian Constitution is not changed…), I refuse the game bad Putin/Russia and good UK/US/France etc. As you rightly reminded us, France did its bits not so long time ago, as US and UK.
I just want to know the rules and what is concealed to us…