The big problem here is that the jobs the MR2 used to do were so diverse, it might have been harder to bin it if they layed all their eggs in one basket.
People are arguing on here now about SAR topcover, The government binned nimrod when the main argument was defence of the Nuke subs, ASW. then they argued that we need it for anti shipping and anti terrorist jobs, overland istar etc.
If you are hinging your argument for an MPA replacement on the need for SAR topcover then you have lost already (prob have whatever argument).
Speak to the SAR crews, and they will tell you whilst they liked nimrod present on long range jobs they would quite happily go without one and very often did.