so I assume Britain received its imaging intelligence from the US as part of five eyes. or else I cannot explain how a nuclear armed country has skipped over one of the main prerequisite for missile targeting for 60 years
Also, even large cities have places which are better to target
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.