Why a 1960s Royal Navy light carrier would use CATOBAR

Would a 30,000-35,000-ton Royal Navy carrier built in the 1960s have been STOVL rather than CATOBAR?

No. In that decade it would have been a small CATOBAR carrier because useful STOVL technology was not mature enough.

A 30,000-35,000-ton British carrier built in the 1960s would still need catapults and arresting gear. Practical shipboard STOVL technology was not sufficiently developed to determine its configuration. At that displacement, the result would be a small CATOBAR carrier with a limited aircraft complement and little improvement over existing carriers. Moving the same proposal into the 1980s changes the answer because STOVL aviation would then be available.