I daresay the reason the Govt are happy to 'gap' the capability provided by Nimrod MR2 is because they know the majority of Joe Public, like some contributors to this thread, have no idea exactly what that capability is and therefore will not miss it.
Sure you can use Albert for top cover and E3 for On Scene Commander (OSC) but unless I'm much mistaken only the Nimrod has the ability to search for survivors, offer top cover and collision avoidance to rescue helo's and act as OSC to control multiple assets during the same major incident.
With a dedicated maritime search radar, AIS, EO/IR camera, multiple V/U and HF comms boxes, PLB location and homing eqpt and a vast bomb bay for ASR eqpt, the Nimrod MR2 really has it all. More importantly, it has a large crew who regularly practice SAR procedures both in the air and simulator. Top cover for helo's can be fairly routine (sometimes not) but I can assure you a major incident requires training, flexibility, and a good working knowledge of maritime SAR assets, Coastguard procedures and what ships can and cannot do. In short it's not something you can just pitch up to and hope for the best.
The Nimrod force has become used to the phrase 'we'll take it on risk' in recent years. I guess a common perception is that major incidents happen once in a blue moon, if at all. I just checked my log book and in the last 6 years I've been scrambled on 8 top cover and search missions and 3 major incidents all of which were headline news. That's just been my crew, there have been more.
However, based on those figures and this Govt's obsession with the manipulation of statistics, that makes it one major incident every 2 years.
Which, happily enough, is the gap between MR2 OSD and MRA4 ISD. Seemples. 