Agree with you on the withdrawl of MR2 5 For 6 Back and it makes some sense, but delaying the operational ISD and indeed the introduction of the MRA4 as a whole highlights that they are very willing to be without MPA. If, after 12 - 24 months nothing major has occurred in homeland waters and therefore there has been no "major" requirement for Nimrod, then it doesn't take a genius to work out what will happen next.
On the positive side, 18 and 27 sqn will have some very highly experienced aircrew to bolster them!