There's also the number actually in use - the UK has four nuclear submarines, one of which is always at sea, so can only ever really launch 1/4 of it's armament

There's also the number actually in use - the UK has four nuclear submarines, one of which is always at sea, so can only ever really launch 1/4 of it's armament. The rest will be sitting somewhere when the big day comes, never to be used.