There is another level to the problem, which is the executability of the JSF program as a whole, given the following facts:
The SDD program is farther from completion than it was (according to official plans) three years ago.
The estimated program cost (R&D + acquisition) has increased by $36+ million dollars per day in then-year dollars since the contract was awarded. (Source: 2011 SAR.)
The program of record calls for US-only JSF procurement funding to reach $13 billion-plus (in 2012 dollars) at full rate, which is much more than all TacAir funding today, at a time when the USAF is expecting to be developing a new bomber and acquiring tankers.
This peak level will be sustained throughout the 2020s. If the operating cost estimates in the SAR are correct, there will be a parallel surge in TacAir O&S as the growing JSF force coexists with older aircraft with aging issue.
(Note: it can be posited that in 2030 we approach the sunlit uplands of a force dominated by mature, but still relatively new F-35s. The question is whether the fiscal pig represented in the above two paragraphs can fit through the budgetary python.)
The US government, Congress and Administration, are committed to further budget cuts, whether as a result of sequestration or as part of a deal to avoid sequestration. If neither happens, the risk to US credit is extreme. Defense cannot be exempt from Federal spending cuts and F-35 cannot be ring-fenced within defense, because it is too large relative to other programs.
Any UK decision must take into account the risk that these factors pose to the JSF program, in terms of cost, schedule and the pursuit of all three variants.