You have no idea how many the UK has.
It is a defensive tool. Invade our land you get it back. You are describing it being used as an offensive tool. Conventional forces fulfill this. If we invaded Russia we would expect to be nuked. If they put boots on our land they would get it.
A few dozen at most. While we do have at least one submarine at sea at all times most of our warheads are in storage or tied up at the dockside while the ships undergo maintenance. I'm not even sure if we have aircraft launched nuclear weapons any more.
This would suggest just over two hundred but we rely heavily on the US to maintain them. Less than twenty are at sea at any one time. For a country as large as Russia, a drop in the ocean.