Thanks OA, BPLT and LO for your replies.
In retrospect, perhaps I should have asked why STOVL for JCA. If STOVL is the right answer then F35B is the only game in town.
It would seem that if the RAF and the RN were choosing in isolation from one another STOVL would not be chosen. Those navies with carriers large enough to operate them choose CTOL aircraft e.g. USN, MN and (once Gorshkov is converted) the Indian Navy. No other airforce (except USMC for its air capable assault ships) which has expressed an interest in JSF is opting for the F35B. So a major reason for choosing STOVL seems to be the "jointness" of the requirement.
LO identifies two very credible reasons for the choice. The first is both historical and commercial so must be right. The second is one I was alluding to in 1) in my previous post but put more succinctly
Another was that the UK was convinced (by things like the fly-by-wire Harrier demo) that a STOVL jet could be truly "amphibious" without the expensive, crew-time-dominating and airframe-life-eating recurrent training associated with cat-and-trap operations.
By following some quite sensible reasoning the UK appears to have found itself in a somewhat uncomfortable position. In F35B it is tied into an aircraft programme where it has little if any control over technical or commercial matters (in particular, cost). The aircraft itself suffers from a number of operational drawbacks compared to conventional FJs. The programme is very risky as weight growth has already led to a fundamental redesign of the structure and not many aircraft grow lighter through their service lives. Finally questions with regard to "sovereignty" are still outstanding.
Is STOVL the right choice for JCA?