GK - I'm not sure that I got my point across.
The US is perfectly capable of developing the F-35B on its own and a UK switch would not have a vast direct effect on the program.
On the other hand, if the B is perceived to be in trouble, or just not worth the added cost, or rendered less important by a change in the role of the Marines, then a UK withdrawal means that it can be canceled or delayed without the Pentagon getting a nastygram from Hilary Clinton, complaining that an allied government has just been made to look monumentally stupid and is now sitting there with two half-build grey cruise liners.