Can't help wondering if it would have been a good idea to let the US finish JSF and then buy some...
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JFZ90,
Yes it would. This is the conundrum that F35's other foreign buyers are getting very nervous about (Australia, Canada and Japan at least). Whatever euphemisms the Pentagon uses to explain deferral of purchase of the F35 for their own forces, the fact remains they could use the space to dump one, two or all three versions of it should testing not shake out or a new show-stopper bring it to a standstill.
While the US will, for now, buy small numbers of airframes (small for them that is), it is not really going to commit to significant buys until 2020+. This potentially puts the foreign carts before the launch customer's horse, which would be very unusual for a project of this magnitude, and would leave them in a desperate situation should the horse bolt.
The UK is in an even bigger bind because two programmes are mutually dependent - carriers and fighter. Contingency planning against this risk must be being considered, in the same way we have seen the USAF, USN, US Marines and the Australians all take at least some steps to mitigate the risk. Publicly, we seem to be focussed on F35 B or C with the associated carrier implications, but it would be be dereliction of duty not to be planning for other eventualities, even if such plans are not reaching the public domain.