The 'Elephant in the living room', regarding JSF, being; seeing as how we are getting big carriers anyway, why not just go for the C version?
I think the correct answer has been listed earlier in this thread by someone else (but I can't be bothered to check)
Steam catapults cost a hell of a lot to run and maintain. Basically we cannot afford them. EM catapults should in theory be more maintainable, but are a bit too risky at present. Hence the MoD decision to go ahead as a STOVL ship but with the option to convert to CV if the EM catapult technology gets sufficiently proven.
I haven't seen the figures so I cannot put hand on heart and say its right but the logic seems fair enough (if frustrating)