The idea of having one aircraft be able to become any of the other variants isn't actually that great. Air Force planes don't operate from carriers, and Navy planes don't need to operate from very small fields.
I disagree, there have been cases where it could have been very useful. For example in the 1980s the RAF were operating the Harrier for the close support role (from austere forward airfields) and the RN were operating the Sea Harrier for air defence, but when the Falklands War kicked off, RAF aircraft were brought onto the carriers. The RAF were also operating the Tornado for long-range bombing and another variant of Tornado for air defence, plus the Jaguar. That's 4 different aircraft that could be merged into a "kit" F35 if it were possible to do so.
Yes, the UK had all of those 4 different aircraft. But the F-35 is primarily a US aircraft, designed primarily for US purposes, and only bought by other nations as fitting their purposes close enough. The F-35B fits UK RN FAA requirements close enough, so they're buying it. The UK RAF has requirements that are better met by the F-35A, so they're doing that.
The F-35B is explicitly the replacement for the Harrier (Sea or regular). The F-35A is designed for both interdiction/strike (it is a "strike fighter"), as well as being able to swing role[0] into air defense. The F-35C is explicitly only for USN requirements. No one else is going to be buying them, because the only allied country that operates CATOBAR aircraft is France, who likes their Rafales.
For all of the F135 being literally the most powerful low bypass turbofan in the world, it's still not enough power to be able to adequetely lift the F-35B without the B being lightened compared to the A. It's not just parts they left off, like the arresting hook, but different parts to shave weight. For example, the weapons bays are smaller. Tails are smaller. Airframe structure is lighter, which reduces durability, and also limits the maximum g-load (22% lower! than the A model). They also had to make the attachment of the wings more difficult compared to earlier plans, because the easier wing attachments weighed too much. The left shoulder of the aircraft are different, because the A model has a gun there that the others don't. These limitations are baked in, and unbaking them would make it too heavy to be able to perform the required role.
[0]: Swing role is more than just being able to do more than one role, but being able to do both of them during the same flight.
The only reason FAA ended up with F-35B is that they were too late with the idea of switching to F-35C, which meant that they couldn't rebuild their new carriers for CATOBAR on reasonable time/money budget.
Meanwhile F-35B exists pretty much only due to USMC to the point that I would be doubtful of any praise for the F-35 if it came from USMC crew. Then it got pushed as option for export, often seemingly presenting F-35B as being as capable as F-35A.
Meanwhile the ALIS without which F-35 is an expensive paperweight turned out to be a very expensive turd, that finally got renamed as ODIN after awarding the new contract to the same group that fscked up ALIS :/
Yeah, the RN screwed up their requirements such that the new UK carriers aren't actually going to be built for CATOBAR, even though they're supposed to be equipped for, but not with, the facilities to do so.
The F-35B is obviously not going to be capable in the same way as the A. However it's significantly more capable than the Harrier, which is the important part for everyone buying it. There is no other current STOVL carrier fighter jet available. And the reduced capabilities may largely not matter to those users, in exchange for the increased capabilities for being able to launch them off ships/very limited area land strips. I think the only nation that is going to not be launching them off ships is Singapore, which is very restricted in land area.
Yeah, the RN screwed up their requirements such that the new UK carriers aren't actually going to be built for CATOBAR, even though they're supposed to be equipped for, but not with, the facilities to do so.
You speak in the past tense but both are actually in the water now! One of them is working up for her first live deployment.
I personally don't believe that CATOBAR was ever an option on these ships, there are too many clues. One is that both cats and traps are enormous pieces of machinery that need to be integrated with the hull and dissipate enormous amounts of heat, they can't be pasted in afterwards, they have to be designed in from the start. The catapults require steam or electrical power for EMALS, the QE class can't do steam and can't generate that much power. Operating cats and traps needs lots of deck crew, there aren't enough people without growing the RN by a few thousand people all told. And finally keeping pilots current for arrested landings isn't feasible if you want the freedom to embark RAF crews at short notice, who have only used runways. Basically BAe said they could do it as a tickbox exercise in the planning stages, but when the government looked at doing it and switching to F35C, it would have cost as much as rebuilding them both from scratch.
I disagree, there have been cases where it could have been very useful. For example in the 1980s the RAF were operating the Harrier for the close support role (from austere forward airfields) and the RN were operating the Sea Harrier for air defence, but when the Falklands War kicked off, RAF aircraft were brought onto the carriers. The RAF were also operating the Tornado for long-range bombing and another variant of Tornado for air defence, plus the Jaguar. That's 4 different aircraft that could be merged into a "kit" F35 if it were possible to do so.