Why an Antarctic expedition ship is not a survey ship

Did the ships used for Britain's 1946 Antarctic expedition count as survey ships?

No: they performed survey work, but their primary role was expeditionary rather than dedicated hydrography.

A ship's primary mission is distinct from individual tasks it performs during a voyage. The vessels used for Britain's 1946 Antarctic expedition gathered survey data as part of a wider expeditionary purpose. A dedicated survey ship instead exists principally to collect recurring navigational and operational hydrography. The distinction prevents every scientific or exploratory vessel that takes measurements from being treated as part of the naval survey service.