Why consider a nuclear earthquake bomb?

Why would Britain consider a nuclear successor to the Grand Slam and Tallboy earthquake bombs?

It could be useful against exceptionally hard targets, but radioactive effects would prevent routine use.

A nuclear successor to the British earthquake bombs offered a way to destroy targets beyond conventional penetration and blast effects. That did not make it a normal operational tool. Radioactive contamination and the wider consequences of a nuclear detonation would sharply restrict where and how often it could be used. The answer separates the technical attraction of possessing a capability from the political and environmental practicality of employing it.