The persistent message here is "we are going to buy P-8s and crew them with Seedcorn."
Fine. Low risk. The alternatives are very different to, and smaller than, P-8/MRA4 and they involve development. And a cynic might look at that and argue that the P-8 is the only solution that the requirements writers, competition managers, industry and MAA can't and won't screw up.
However..
You're paying a lot to support US missions that UK does not need (ASuW, APS-154) and US technology (radar) and more. You need to buy US torpedoes and (probably) sonobuoys. With a handful of jets to the USN's 100+, you are locked into US support and (importantly) the upgrade path, which you cannot fall behind. (See AWACS, impending obsolescence of.) It is a big jet and the currently favorable support economics will take a dive at mid-life as the commercial fleet retires the 737NG platform.
And all this without a competition...