But if the UK openly and officially declared war to Russia, then Russia won't have a choice but nuke HMNB Clyde and destroy 75% of British nuclear arsenal and then blackmail the UK in peace in Russian terms.
If the UK already declared war, he has no choice between war and peace. He have choice between situations in which he, after the proper preparations, attack first and then demanded the UK to surrender (in which the UK can retaliate and destroy not Moscow, but may be Saint-Petersbourg or other large city):
- optimistic scenario, Russia doesn't suffer civilian losses at all,
- realistic scenario, Russia suffer some thousands of civilians casualties;
- pessimistic scenario, Russia suffer lesser than 1 mln of civilian losses.
and between situation in which the UK attack first with all their three SSBNs in the moment of their choice and minimal Russian preparedness (of course, after such attack Russia will totally annihilate Britain, but who knows, may be British Toffs already have shelters in New Zealand and they simply don't care about ordinary Brits).
- optimistic scenario, Russia suffer few million civilian causualties;
- realistic scenario, Russia suffer five million casualties;
- pessimistic scenario, Russia suffer twenty million casualties. (Plus there is possibility of escalation).
Facing this choice Russia simpy won't care about position of China, India, America or whoever else. War is war, and other countries may be allied, neutral or choose escalation and try to do something stupid. (That is exactly why in the described situation Russia will eliminate British nuclear forces immediately).