Navy Matters - CVF
The decision process weighed the RN's existing STOVL training and experience, and the lower ship construction and running costs of a STOVL carrier, against the significant aircraft weight & performance benefits of CTOL operations, the ability to cross-deck operate with US and French carriers, and a greater possibility of landing-on damaged aircraft.
I believe a further factor was the ease of qualifying STOVL pilots on carrier ops and maintaining their currency as opposed to qualifying and maintaining for CTOL carrier ops. That will allow the whole of the F-35 joint force to be available for carrier operations when required.