I reckon the UK would launch if the US did. We don't really have a fully independent nuclear deterrent so we are likely tied in to the American response anyhow.
Yeah this, I think the deal is that we effectively need American co-operation (permission?) to operate Trident. So potentially could hold off joining in, but would need them to enable us if we wanted to. We would obviously get involved though.
I don’t think Russia will launch a nuke (tactical or otherwise) at Ukraine, as it really doesn’t serve a purpose. The reason why the yanks/NATO don’t have tactical nukes is that their conventional arsenal serves the same purpose without the necessity of using “WMDs” - MOAB being an example. Therefore, Russia would only invite a conventional response with no ability to truly escalate beyond an attack on NATO countries that’ll only invite a relatively immediate loss. A strategic nuke at that point would be game over for everyone - fecks the point of being a trillionaire Russian leader if you can’t do anything and your nation doesn’t exist. Lastly, China obviously does not want a nuclear war.
I really hope sense prevails sooner rather than later and Russia somehow reverse ferrets and comes to its senses. The obsession with a multipolar world (i.e. Russia is a superpower) is holding the world to ransom by diverting time and energy to potential war. Far greater issues at stake.