Whilst the sentiment about taxation is correct, there are rather interesting offset percentages at play for JSF - at least from the numbers presented.
If the UK really has 15-20% of the work on all JSFs, it could well end up free to buy (if not support) if they make 3000 and we only buy 48; 48 aircraft would only represent 1.6% of a total buy of 3000.
I would be very impressed if those percentages applied across all 3000 - I suspect UK content on the F35A for example must be much lower than 15%.
Thinking about it, key UK bits are on the smaller volume variants - liftfan & other STOVL bits obviously is only on B, refuelling probes are only on B/C, etc.