I have no more knowledge of the inner workings of the MOD than the average man on the street, nor do I wish to. However, I don't believe the RN has its own pot of money, especially a spare £1Bn, to go out and buy replacement MPA - let alone a funding stream to crew and maintain them for the next 20+ years.
Surely any such intent has to go through the normal procurement cycle (even if it is crap?) go into the core budget, etc, etc, and will require ministerial approval at some point.
Or has the RN found a spare £1Bn down the back of the MOD sofa?