This question is being dealt with at the moment. If rumours are anything to go by, the RAF want to keep the putative money for an MPA, but are unwilling to commit to an MPA. This is not to say there aren't lots of RAF guys rooting for a future MPA capability, but the RAF as an entity isn't convinced.
This follows some fairly bad behaviour by the RAF in PR11 regarding money in budget saved by deletions of capability, so unsurprisingly the RN are pushing very hard for the entire MPA capability (and associated funding) to be transferred to 1SL's budget, and he is then held to account for the capability he chooses to invest in.