Atleast the Sentinel is in service now...
P-8 Poseidon seems the logical way to go in regards to being a brand new airframe with ample spares available off standard B737.
Buy a few more and use them for replacements of the Nimrod R1 also.
Commonality between two variants as seen already with the Nimrod albeit less of a maintenance issue and a brand new start.
Or, it will go the same way as the Tankers, end up leasing said aircraft from an airline or consortium and being even more delayed.