Why relabeling carriers did not create treaty tonnage

Could Britain exclude Argus and Hermes from treaty limits by classifying them as something other than fleet carriers?

No. The treaty counted total carrier tonnage, so changing the ships' subclass labels would not remove them from the national allowance.

The treaty controlled national aircraft-carrier tonnage, not a navy's preferred subclass label. Reclassifying Argus or Hermes as escort carriers therefore would not create unused fleet-carrier allowance. The hulls still performed aviation roles and their tonnage still counted against the carrier ceiling. The exchange separates operational categories such as fleet, light, and escort carrier from the legal accounting category used by the treaties.