The number of available airframes is small, even with MRA4 numbers having been slashed. I’ve heard (but can’t confirm – pure hearsay) that examination of the stored airframes has found it would be difficult to find enough airframes with requisite parts sound enough to re-use even for the original MRA4 requirement, & that means just 9 more than now planned. If true, then it would be necessary to make new Nimrod fuselages from scratch. However, that would not be an insuperable problem. They’re already making most of the airframe new. But we’re talking small batch production, hence expensive. Luckily, the additional development cost would be minor, compared to what’s already been spent, & any new-build fuselages wouldn’t suffer from the sloppy tolerances which have caused such difficulties matching up old Nimrod bodies with new, precisely machined by computer, parts. It’s probably the cheapest option, overall.

Just to add to that excellent post, a fews years ago BAe did a study into making new Nimrod fuselages, IIRC it was related the US requirement for a new maritime patrol aircraft (now the P-8). They have already built new wings and studied new fuselages so it all sounds rather feasible and probably not overly expensive, in fact if you chopped out a lot of the Maritime gear you could probably save a fair amount of cash. You would not even have to integrate that many weapons, Storm Shadow for long range stand-off ability and some PGM’s for places like Afghanistan.