Why Britain might still choose the F/A-18

Would a similarly sized British aircraft make the F/A-18 unnecessary for a CVA-01 air group?

Not necessarily; another national design would cost money to develop, while the F/A-18 supplies a proven fighter, an American partnership, and a path to the Super Hornet.

The proposed British alternative may have dimensions close to the F/A-18, but it would still need to be developed and adapted for carrier service. Britain could struggle to afford another bespoke aircraft alongside the strike program. Buying the Hornet would combine an Anglo-French Jaguar partnership with an Anglo-American fighter partnership. That choice also opens a later path to the Super Hornet, making industrial access and long-term development as important as the initial airframe's size.