Carrier treaty tonnage versus Royal Navy hull numbers

What treaty change would allow the Royal Navy to build a larger Illustrious-type carrier?

The total carrier allowance would need to rise enough to preserve at least six ships, not merely permit a larger individual carrier.

The discussion contrasts five 27,500-ton carriers with roughly six ships of 22,500 tons under the same 135,000-ton allowance. A design near 25,000 tons would require a larger total allowance if Britain were to retain six hulls.