UK would nuke Russia most likely in retaliation. They have 4 subs which should be able to carry 48 nukes each (always at least one of them is in water, but I think now more if not all are in water). When a prime minister is elected he writes a letter to the captain of those ships that can be opened only if they cannot contact the chain of command for 4 hours.
Apparently the letter can have 4 options: retaliate, go under US command if it exists, go towards Australia, and use your own judgement. So far, no letter has ever be opened.
Who knows what Boris has written but obviously retaliate is the safest option. So if is retaliate, you can expect Moscow and another 50 or so large Russian cities being burned in return.
And that is without the US retaliation which will obviously come.