Still on here (but mostly been lurking).
I do wonder though, if this is not precisely the correct role for Britain. Our job is to take the "extreme" West line - the things that ought to be said, but nobody else can say them. the US can't, it is too important, so it needs to be measured and careful. None of the countries that border Ukraine can, they are too vulnerable. Germany is too dependent on gas/oil and won't. France could, but won't. We are far enough away, and can always play bad cop to the US's good cop (when negotiating, usually the bad cop is the less senior person, so that the good cop can come in and overrule them if/when they judge it appropriate).
Given our role in spotting the issues and arming Ukraine (we didn't have to sack our heads of intelligence, like the French, we were the ones saying 1) that Putin was going to invade when everybody else was saying nah, and 2) that the Ukranians would fight, when everybody else was saying 3 days and it would all be over), we are the logical one to make the more extreme demands at this stage.
Ultimately everybody in the West and Ukraine wants Russia out of Crimea and Donbass. Whether it is possible, who knows, but somebody needs to stake out what are the fully desired options, so that the others look more reasonable when they concede. If that is Britain's role, iis it not a reasonable one for it to play? Everybody knows that we can't go in and make it happen, we are just not strong enough and we are a bit to far for reprisals (it would be mad to nuke us only, as the US would respond and nuke Russia, so they get the little fish and the US gets the big one, but short of nukes, what else can they do, we are not Poland).
So just wondering if the bluster is not exactly what is needed from Britain, so everybody else can sound more measured and reasonable and can potentially negotiate?