If the F35B is cancelled can the UK claim back any of the investment in the JSF from the Americans? Would the MOD have been silly enough to sign a contract that let's the US off the hook for all the billions of pounds invested without providing the end product?
Or could the UK sting the US for the cost of converting the carriers to cat and trap ships?
Almost certainly not. The Tier 1 partnership bunce buys you a seat at the requirement definition table and (some) access to the technology as it develops. In essence you have a vote in the direction the programme goes in and you get some of the technical output. But it's all development - if you want the aircraft you have to pay the production cost. If the programme is canned, you don't pay the production cost.
However, despite the somewhat frenzied anticipation in some quarters, the US does not have a plan B. Simpson-Bowles as I understand it envisages some sort of F16 restart (configuration / block unknown) and an extension of F18E/F production.
Whether these actually meet a requirement or not does not appear to have been considered, which is a bit disappointing, especially considering the effort that has gone into the JIRD and JORD over the years. If I were an LM exec, I'd be lobbying my Congresstypes, suggesting that all sorts of legal challenges might be forthcoming unless there is demonstrable evidence that the requirement has changed. That could just run and run......
In essence, the detail of any Plan B is ephemeral at best and largely consists of people going "I like not this aircraft, bring me another aircraft", without the slightest idea of how that is going to be achieved, particularly for the USAF.
There is also the slight issue that (one assumes) the USMC still requires some sort of jet that can fly off the LHD/LHA. Whether it is day 1 stealthy or not (which is probably the only contentious bit) is to a degree irrelevant. The last time I looked, there isn't a frame in production anywhere in the world that would provide that capability. If that capability is required then you're looking at either
re-starting a production line that's been dead for the best part of twenty years or designing a brand-spanking new STOVL jet. Neither is a particularly sensible choice.