Doubtful that they would retaliate directly on London. Maybe taking out North Sea oil platforms would make more sense.
Depends. What if some Brit coordinated and supplied missile hit the Victory Day parade right in front the viewing stand and killed a few hundred people?
Eventually, the Brit will get some lucky shot in and the Russians will logically decide that they lose nothing by sinking a Brit carrier, or hitting some critical Brit sites.
Or.... the cables to North America are cut in 3 places simultaneously.
Nobody is backing down. And has really bought into “they wouldn’t dare”.