RN made the decision to go F-35B about 20 years ago. The 2004 Defence White Paper outlined the way ahead for the carriers as VSTOL.
The concern was this: if the EMALS system had problems, then the Prince of Wales and Queen Elizabeth would be white elephants. F-35C offered more range, payload and simpler maintenance, but the all electric architecture of the ships required EMALS to work, and it was still in development at the time.
So, the decision by the RN was for VSTOL, and F-35B. No EMALS needed. Further, if the Airplane slipped in delivery date (and it did, just a bit) then RAF Harrier GR-9s would fill the “gap” (mind the gap…) between the retirement of the RN Harriers and the delivery of the F-35B.
Ultimately, the GR-9s were retired early, too, and there was a gap, but the RN is very happy with the current performance of F-35B.
Unless the RAF has a stealth multi-role fighter, they fall behind other NATO nations and the RN.
That simply won’t do. So, this isn’t “instead of” anything, it is the RAF keeping up.