Could Russia defeat Britain in a war?

Could Russia attack and defeat the British military if no other country gets involved?

Without nuclear weapons, Russia could wreak havoc with cruise missiles. However, Russia lacks transports to deliver sufficient ground troops, heavy weapons, fuel and ammo to fight the British military. Whatever they have, the UK could defeat with submarines, shore-based artillery, torpedoes, anti-ship missiles and air defense. The total size of the Russian military is irrelevant, what matters is the capabilities.

With nuclear weapons, Russia could technically obliterate the UK, but would then suffer a second strike and lose most of its large cities, after which electricity, fuel and food distribution would collapse, and so would manufacturing, along with the entire economy. Note that UK submarines on patrol can launch nuclear strikes at Russia independently of any other countries if necessary. The question assumes that no other country helps Russia, leaving the Russian population doomed to starvation (recall how the US sent food aid to Russia until 1999 or so).

Moreover, even if no other country gets involved on the UK side, Russia would not necessarily know this. Without knowing this, Russia would expect a NATO response (according to Article V), and that would be devastating to Russia any way you look at it. So, Russia would not commit significant forces to an attack on the UK. Without a significant commitment, an attack would not succeed.

Source: QUORA