It depends on who nuked U.K and why – what warranted such an attack? But it is hard to think that US would not come in to offer solidarity to U.K even though U.K can send nukes independently of US. Considering both countries are long time allies and members of NATO, such an attack on U.K would be answered in coordination with the U.K government and NATO.
It will dawn on U.S government that any country that send nukes on U.K would fancy the chance to do the same to the U.S. Expect U.S government to invoke Article 5 of the NATO treaty- and offer support to their closest ally. Given that both are NATO members, that would be by “nuking” whoever was responsible alongside the British nuclear retaliation.
If Russia did it, Britain would nuke them back which would trigger Russia’s automated nuke every possible future enemy system. That would mean Russia would nuke the USA, China and a load of other countries and bring them into the nuclear war. One of the countries at least would nuke Russia as well until it is a lump of glowing rock.Currently on its own Britain can destroy 240+ cities with nukes.
