Hmmm, a moment of clarity.
SDSR came up with Harrier, CVS and DPOC as savings set against the cost keeping the Tornado (getting rid of it would have saved £600 million more over ten years) and variant change to F-35C.
18 months on it turns out (caveat - this is a rumour site) that the Tornado ate the £1 billion saved from DPOC (its successor - oh the irony - can't afford to buy the new aeroplane we spent all the money on the old one), which was actually needed for QECV conversion. Hence the motivation for F-35B variant re-selection. Yes, makes perfect sense!
That and its easier to land, you don't need pilots and deck crews to be out in the USA, it's similar to a helo so RN helo types can supervise it....so RN types don't need F-35B cockpits...all so clear now.