There is a capability gap here that will need to be covered, but it is clear that is going to be at least one new government away.
How it will be addressed is conjecture, but it may not be in a single asset a la Nimrod. It may be in a range of different systems which could include 'drones' - surface and sub-surface as well as airborne, and maybe some sort of mid-sized manned platform on a smaller scale than Nimrod, of which many types are springing up in view of the capability requirement, but hard-pressed budgets that don't stretch to the full monty.
A few years from now Poseidon will be known quantity, and may or may not be cost effective for the UK. US unit price appears to be much lower than MRA4, but I am not sure if it is a fair comparison. Given the proposed unit numbers it ought to be competitive and with 20/20 hindsight it might have been better to join that programme than fund a customised development of a very few units - a perennial source of very expensive procurement decisions.
A maritime nation lacking MPA capability, or complementary suite of capabilities, is a situation best rectified as soon as possible.
LF