One side has a robust and updated civilian civil defense.
One side does not.
That is why nuclear deterrence is a dead concept—Ozzie and Harriet stuff.
No amount of civil defence preparation would change the fact that all Russia’s major towns and cities would be reduced to radioactive rubble and the vast majority of their citizens dead.
There might be some people in Siberia or underground bunkers that avoid relatively unscathed in the immediate aftermath but life thereafter for those relatively few would be a ghastly struggle and it’s hard to see how it would be worth reducing Russia to that state just because Britain itself would be annihilated entirely. After trying to fend off starvation throughout a long nuclear winter many of the survivors would come to envy those who died in the initial strikes.