Why Britain did not field twin-engine carrier aircraft earlier

Did treaty-sized carriers prevent the Royal Navy from considering twin-engine aircraft?

No; the larger obstacle was the indirect Air Ministry procurement process, though later ships required lift modifications for twin-engine types.

Treaty-sized carriers did not make twin-engine aircraft impossible. The Navy could have designed ships around them, but requirements passed through too many institutional layers to sustain the necessary technical conversation. Single-engine development therefore dominated before and during the war. Postwar lift modifications made twin-engine types more practical, although jet propulsion soon changed the problem again.