As a Canadian ex-maritime patrol pilot and aerospace engineer who had some peripheral involvement with RMPA and BAe during the solicitation of interest phase, I wish MRA4 the best.
I also agree that reducing the fleet is a bad decision, just when the project appears to be emerging from its cloud. MPA still have great generalized capability for surveillance, whether subsurface, water surface, or overland in controlled threat environments.
During that early time, i.e. 93-94, the BAe systems engineers I dealt with were quite nervous that MOD would drive them to a new wing, which they believed would blow up the risk, cost and schedule. Clearly they were right.
On the other hand, I don't understand the scathing comment about MMA. The 737 is a good aircraft with reasonable payload/range for the role and reportedly excellent handling. The mission system presumably will take into account lessons learned during MRA4, for which Boeing also produced the mission system.
Bomb bay capacity (an area where Nimrod has no peer) may be one weakness on a 737 solution, but otherwise I believe it would be at least competitive with the MRA4 airframe/engines and in some respect superior. It also has the advantage of starting from newer internal systems (fuel/hydraulics/services etc.) than Nimrod.
A P3 would have been a lower capability solution. The P3 is derived from the '50s Electra, i.e. contemporary with Comet, and is not a modern design. It also has a very stiff and short wing, with consequent poor low altitude ride and limited manoeuvrability. It doesn't even have anti-skid! Canadian Aurora pilots generally try not to use the brakes on landing on slippery runways due to the risk of blown tires. Good thing those large Hamilton Standard paddles are good tools for slowing down.
The two P3 variants proposed for RMPA were:
1. Used/refurbished P3Bs with a new mission system - clearly less capable than MRA4, and
2. New P3s with mission system derived from USN standard. USN standard at that time was an only paritially integrated system with an outmoded architecture and which had limitations in the mission computer. Not nearly as advanced as MRA4.
If either P3 option had been chosen, the RAF would perhaps have had a solution by now, but one with significant shortfalls compared to MMA.
MRA4 has taken too long and cost too much, but there is no point in dwelling on the reasons for that now: It has the potential to be a superior platform.