Agree in principle but requires clarification in related areas of the emerging Local Plan.
The proposed policy captures the importance of restricting development which could lead to the irreversible loss of the best agricultural land (Grades 1, 2 or 3a) and is reinforced by the concerns raised in response to the First Conversation, regarding carbon sequestration, biodiversity and our ability to meet our food growing needs.
The proposed policy is understood to follow Policy NH/3: Protecting Agricultural Land, in the current adopted South Cambridgeshire Local Plan 2018.  Section 1a. of adopted Policy NH/3 states:
Planning permission will not be granted for development which would lead to the irreversible loss of Grades 1, 2 or 3a agricultural land unless:
a. Land is allocated for development in the Local Plan;
Clearly, it would be inappropriate to allocate agricultural land in the Local Plan for developments that are still under investigation, such as the proposed Waste Water Treatment Plant relocation. However, in the context of the proposed North East Cambridge development, the emerging Local Plan should support the principle of adopted Policy NH/3 by clearly identifying the corresponding agricultural land and Green Belt cost that would require allocation in the Local Plan as a consequence of the proposed North East Cambridge development to allow a comprehensive assessment to be made.