[mrguyorama]

>Presumably part of the nuclear doctrine is to take out your enemies nuclear launch sites before they launch

It is not. The UK, like most nuclear armed states, has a "No first use" doctrine, meaning its nukes exist only to retaliate, to turn cities to ash.

Whoever is nuking you, to the point that you use those retaliatory nukes, might be targeting your weapons, but they are on submarines in friendly waters, so not likely to be destroyed before they can launch.