A couple of relevant points from Tony Blackman's book.
The Nimrod was no 'bodged up Comet' from all accounts the MR2 was a superb
aircraft in its role, from its high speed transit ability to the flexibility it offered
with the huge installed bomb bay and the variety of sensors on board, it seems
to have been a very effective platform.
The AEW version really was a disaster and the technical problems were never worked
out.
The MRA4 on the other hand was 'ready to go' all issues, such as bespoke fittings
had been worked out and the sensor suite was first rate, Tony claims it was a better
aircraft than the P8 and it's performance was outstanding, also it would have kept
that huge bomb bay, far bigger than the P8 along with wing hard points it could
carry an impressive amount of weapons and / or rescue gear.
This is all from his book, i'm no expert but it appears the real reason the MRA4 was cancelled was a temporary budget shortfall, nothing to do with technical shortcomings.
It also had a MAD boom as standard, unlike the P8 which will end up costing more anyway.