Henra asked where does the optimism come from that B Version still equals 2 active carriers.
I don't think there is any suggestion that there would ever be 2 carriers active at the same time, rather that because both would be usable without conversion we would cycle them in and out of high readiness, meaning we'd have one carrier available 8 years out of 8. Given how difficult reversion to B will be politically (the Guardian has helpfully reprinted all Cameron's statements about B vs C at the time of the SDSR) I'm kind of hoping that the Govt will cite the ability to alternate between both carriers as one way of sugaring a difficult pill.