Really? And who will make Saint-Petersbourg disappears? America? Will they make it without even a minimal preparations to save their own people - like, say, evacuation, sheltering, counter-force strike or something?
Ok. Let's play the game.
1) Britain, in attempt to nullificate US peace-searching efforts, without American permission and even in violation of American direct restrictions, attacked Russian Baltic Fleet and officially declared war to Russia.
2) Russia nuked HMNB Clyde with two Vanguards and destroyed the last remaining Vanguard in sea. Which makes the UK lack of nukes. Then, Russia suggest to UK unconditional surrender.
3) UK agreed. Will the USA attack Saint-Petersbourg in described circimstances and if they will - why?