What would Ukraine do with nukes? Threaten to nuke Moscow? Knowing their country would become radioactive glass in response? Russia would have no reason to assume Ukraine would be insane enough to use them over losing a few regions, and so would have no reason to take them much into account.
As a side note, Ukraine actually did have nuclear weapons post-USSR. Then, Ukraine agreed to give them up, in exchange for clauses in treaties where Russia guarantees its territorial integrity.
So now, after Crimean acquisition, we know that the Russian signature in nuclear treaties is worth nothing.
Ukraine is not part of NATO the UK is this makes a difference to our defence requirements. NATO might be stronger if the UK took half the money to be spent on Trident and spent it on fighter jets (and the other half on not cutting junior doctors' salaries or cutting taxes or whatever).