There wasn't any P-8 when MRA4 began. The USN had cancelled P-7. There was no off the shelf MPA available. P-3 & Atlantique were out of production.All the existing Nimrod airframes differed in various ways from one another, to the extent they were almost handbuilt - realistically, the cheapest way forward for MRA4 would have been to go for new build throughout, given that they had new wings and needed new bodies.
But of course, if that'd been asked for, someone would have just ordered P8 or whatever,
Ian
The logical options were re-opening the production line for P-3 or Atlantique to build some just for the UK, refurbishing ex-USN P-3s (all offered), refurbishing Nimrods, or new-build Nimrods.
What was chosen was a new-build Nimrod derivative incorporating fuselage parts from old airframes.
The chief engineer on the project refused to sign off the risk assessment, because it was so flawed, & was sacked for his honesty. This didn't come out until too late.