I'd suggest that Nimrod (and Sea King ASAC 7) was used in Afghan and Iraq because it had a capability that was as useful over land as it is over the sea. It may not have been designed to do that particular role but when it works then why not use it?
My first contact with the MPA world was on long trails to the US and Malaysia where the Nimrod provided SAR for our jets. Very useful and comforting it was too to have that sort of asset close on hand if the worst happened to our single engined aircraft.
In the UK our Armed Forces have always had to make best use of any and all assets available and both recent theatres have ben rather large for Sentinal (which was designed for the job) to take on alone.
As far as I know the Nimrod force still provided all the usual benefits of an MPA to the UK and other areas (V boat protection, long range SAR, over water ISTAR etc) at the same time as Telic and Herrick. For a maritime nation, which we are due to the fact that there is water on all sides, to not have an MPA is very odd. It does not matter who owns it, and there are both dark and light blue already involved in P8, it is a capability we need if the Treasury will ever let us.