G3 hulls as Washington Treaty aircraft carriers

Could Britain be prevented from converting G3 hulls into carriers if its other conversion candidates were unavailable?

Probably not; with other conversion hulls unavailable, the G3s would be Britain's natural treaty choice and were incomplete enough to become almost purpose-built carriers.

If the three Courageous-class ships and the unfinished Admiral-class hulls were unavailable, G3 hulls would be Britain's available carrier-conversion candidates. There is no convincing objection offered to Britain choosing those hulls. In this scenario, construction has barely reached the keel, leaving most of the ship to be designed around aviation. That would make the work unusually straightforward and the resulting carriers close to purpose-built ships.