It would probably require Britain being about as dis-honest as everyone else about Treaty Limits?
Something along the lines of working in 3000 Tons of "Improvements" into the original Illustrious design and "accidentally" miscalculating displacement so the ship came out 10-20% overweight.
So that they came out more in the 31,000+ Standard Tonnage range in 1936, at least!?
It wasn't that bad. The original Audacious class design (note the OTL Ark Royal was NOT the Audacious design) came in at a bit under 24,000t. With a treaty limit of 23,000t this isn't bending the treaty by much.
Granted by the time they'd built it it would be more overweight, but by then is war and who cares?