Contractually, the UK's industrial participation in JSF is unrelated to offtake of aircraft.
The only thing on which it is dependent is BAE's ability to continue to offer 'best value'. BAE Samlesbury is already 'tooled up' and making rear ends. Someone else would have to spend huge amounts to compete, and then to offer a better value price. Even more problematic is the fact that BAE Systems have (they tell me) had some design input into the rear fuselage. Is there thus some UK IP in there? The very fact that there is that possibility further complicates the process of finding a new rear fuselage supplier.
In any case, you don't simply withdraw. Because of lengthening timescales and ITAR problems, you regrettably reorientate your JSF procurement, delaying the buy to beyond 2012 and re-framing F-35 as UK FOA or somesuch, and as Tranche 2 of the JCA requirement, with a marinised Typhoon taking over as Tranche 1 of JCA.
The US GAO love this, because they want to delay the programme anyway.
Once the Marinised Typhoon is in production, and once BAE's position in JSF is unassailable, you cancel the residual UK JSF buy.
Once we're beyond LRIP, BAE will be impossible to replace.
Bwahahahahaha.