Yes Russia could annihilate Britain but do what? How does that stop it's more powerful enemies like America or NATO countries? How does it prevent Britain's nuke subs from unloading their trident D5's all over Russia after receiving commands from elsewhere?

More importantly how does it prevent nuclear fallout from the melted down reactors in Britain from blowing across into Russia?

Conclusion nuclear war is suicidal and monumentally stupid, and willingness to use nukes or doomsday weapons shows that you are weak insecure and terrified so you choose to swipe the chessboard off the table rather than play.