I've heard that MRA4 didn't (& it's said it was feared it couldn't) meet safe operating criteria. Dunno if that's true. It's public, & not denied, that the bloke in charge of risk management refused to sign off the project, back in the '90s, & got the choice of immediate (early) retirement with a gagging clause, or sacked with prejudice, e.g. no pension. He went public after it was cancelled, reasoning that BAE was probably not going to retaliate at that point. I think everyone above him had retired by then. IIRC his reasons for not thinking the risk case credible all turned out to be correct.
The idea of a new MPA based on Nimrod wasn't a bad one in itself, IMO, but deciding to do it by scavenging half the airframe from 1960s hand-finished aircraft which you hadn't inspected before deciding to go ahead, & planning to combine those sections with uniform CAD structures . . . doh!
I think they should either have bitten the bullet & built a new Nimrod-based aircraft, or turned the A320 into an MPA. The latter would have had the advantages of a good chance of export sales (albeit some would probably have been "We'll pay for your airframe mods but we'll put our kit in it", e.g. from France) & being able to piggy-back on the world-wide Airbus support system. It should have been a damn sight cheaper, too.