The UK had already committed to paying for the F-35B test articles. They're needed for the program as a whole, and if the US had to pay for them it could well trigger a Nunn-McCurdy breach - that is, where the projected costs have increased that the entire program has to be reviewed, by law.
Exactly. But the MoD was also suggesting last year that it was planning to use the test models as its own way of deciding whether the F-35B would be suitable.