G3 and N3 construction makes dockyard expansion a 1920s priority

Would completed G3 and N3 classes force Britain to spend the rest of the 1920s upgrading naval infrastructure?

Probably yes, because their foreign responses and future successors would require larger docks and support facilities.

The ships themselves would not be the end of the program. Their size would expose limits in docks, yards, maintenance facilities, and other support infrastructure. Britain would therefore spend much of the later 1920s preparing its industrial base for the responses those ships provoked abroad. The watch point is useful for seeing capital-ship competition as an infrastructure race as well as a contest in armor, guns, and speed.