We did the whole debacle of B to C and back to B, when it was quickly realised just how expensive a conversion of both QEC would be. So, on the basis that the C variant carries significant additional weight solely for conventional carrier ops, it is totally dead weight when you can only use it on another country's conventional carriers. The margin of fuel the C has over the A would be largely negated by the additional weight you are carrying but never going to use for take off or landing. Change the QECs to conventional carriers? That's a different matter, but the costs go through the roof and the UK will inevitably end up with less total aircraft as a result of yet another impassioned U-turn.
So, look at the more important issue. The RAF has a requirement for an addition to its Combat Air "force mix" between 2020-2050. It's called FCAS. Now it isn't necessarily an F-35 but recent interviews say it might likely be. Therefore, why are we even debating a sea-based aircraft for a requirement that is entirely separate from the original one that has resulted in the F-35B? We are buying and operating a load of F-35B from QEC for the next 30-40 years under that requirement. This one is separate.