LO, Others,
Perhaps I can help a little here.
The CALF study involved not only DARPA but at the USN. While it looked at Harrier replacements, and by inference RN requirements, the UK wasn't formally involved.
The UK MoD was certainly sighted on JAST, but again in no more than an unofficial 'sighting' role. The first UK person to formally join the project in Washington was an RN Cdr AEO in around 1993/4. Further personnel joined later in the 90s. UK input to JSF requirements started around 1997 or so, with a UK RN pilot joining the JPO. As far as I know, the UK MoD didn't formally 'join' the JSF programme (i.e. commit money) until 2001, when it signed the STOVL MoD for the development phase. BAES had been part of the early stages of JSF, teamed with NG as one of the three designs competing for the 'X-plane' phase. When they lost out, they joined up with LM. The UK MoD signed up to the post-SDD MoU in 2007.
As I posted earlier on, the US were extremely keen to get BAES in the programme for their unique knowledge and experience of STOVL propulsion integration and flight control.
The main thing I'd like to see recognised one day is the great work done by knowledgable and committed service engineers and aircrew (and BAES engineers) who kept the US/UK lines open through the late 80s and early to mid 90s. their unsung work is the reason the UK was able to get its special 'Tier 1' partner status on JSF. Some of them should have got gongs. Most of them didn't.
On variant choices, I was fairly familiar with the fuel tank layouts, and I can state with some confidence that the main reason the F-35C has more fuel than the A is the bigger wing, with a bigger wing fuel tank. The only reason the C has this sized wing is purely to get on board the CVN at a fully controllable 138 knots or so. Yes, not having an internal gun also helps.
The internal gun did impact tank capacity on the A model, but hot half as much as the boom refuelling receptacle, which occupies a large portion of the main centre fuel tank on top of the wing. That receptacle also adds a lot of weight, as a lot of it is plated in steel to withstand boom strikes. I seem to remember that there was a study done into getting the USAF to adopt a probe refuelling solution (might have been by RAND as well as the GAO), but the USAF weren't willing to change from the boom system. Good reasons were offered, but it aded a good whack of costs to the design.
Interestingly, the original 27mm Mauser cannon installation would have had less impact on the internal volume and external drag. Sadly, US politics won out (as well as an understandable desire to reduce the technical risk associated with a 27mm linkless feed) and the 25mm Gatling was substituted. Heavier, and larger internal volume. Shame.
I am quite certain that somewhere in LM and the DoD, there are schemes for a 'big wing' A model, that would have a C model wing. I'd expect such an aircraft to have a very useful range indeed.
Hope this stuff is of some passing interest. Best Regards as ever to all those who play their parts along the way,
Engines