" For the purpose of the discussion, why should the RN operate Nimrod? Why not the Army?"
Well old bean... the Nimrod is a maritime patrol aircraft. Maritime means 'to do with the sea'. Navies traditionally hang around on the big blue wobbly thing, which is where Nimrod is designed to spend most of its time flying above, generally trying to blow up submarines, gather intel etc. Perhaps given that the army tend to spend a lot of time in muddy fields (ie a long way from the sea), they would perhaps not be best placed to look after an MPA?
Maybe like the US and most other countries, we should consider that we're not so different after all and put Nimrod in the place where it logically belongs?