There's faulty logic here: "the last extremity". The whole point of a deterrent is to deter, not some kind of last gasp that doesn't actually save anyone or anything, which brings back the point I already made about the use of nuclear weapons will always be controversial.
America's use of A-bombs was a deterrent measure, the logic being that thousands die so millions may live through Japan's early surrender. If Ukraine had nukes, then its leader would have to gamble on the support of the West if they used them. If Zelenskyy had approached say Biden in secret to ask if he would have his support in such a scenario, Biden would have said no, and I think most Western leaders would have said the same (right or wrong). Therefore Zelenskyy would have had to just roll the dice and act like Putin was calling his bluff and then just nuke the bastard.
If Putin attacked the UK tomorrow, Starmer would not pull out the nukes. If Putin nuked the UK, then yes, Starmer would likely respond in kind, but that's about the only scenario that wouldn't be a controversial use of nukes, and so therefore the likes of Russia have a lot of rope before they have to start worrying about nukes, especially now that the US isn't going to have anyone's back except those who either have bought Donald's support or is someone he idolises.