Dreadnought's successor is likely to be evolutionary

What does it mean to plan a successor to the Dreadnought-class submarines?

It probably means an improved Dreadnought design built around the next reactor, because SSBNs remain Britain's most practical deterrent platform.

Design continuity is the least risky and least expensive route. Dreadnought evolved from Vanguard around a new reactor, and its replacement would probably evolve in the same way. Britain's limited land area makes fixed or mobile land-based ballistic missiles difficult to hide and politically contentious. Unless the country abandons submarine deterrence, another evolutionary SSBN design remains the probable choice.