That 'mission area' is of course of an AEW version, not a proper Shackleton
at all
You will find that the Mk 2 spent more time in service as an AEW rather than as the MR variant (17 years maritime: 20 years as AEW). Seems to me that AEW is therefore the most important role for the airframe (as also evidenced by SDSR but that's a different thread).