When the UK reinvests in MPA capability it will inevitably be a two or three tier system. ACAS intimated as much at DSEI last year by stating that a future capability would have a manned and unmanned component. Given the prediliction for the UK military to have the shiniest toys the P-8 seems a given; we have trained crews in place and have allegedly made discreet enquiries re costs / availability ahead of SDSR. But it's expensive to buy and fly. More pragmatic, perhaps, would be a wide area oceanic surveillance capability, constantly airborne, provided by a small fleet of Triton/Airlander style UAV/Optionally manned platforms with say 6-8 P-8s on call for reactive tasking, SAR and 'end game' ASW. The third tier remains the RN Merlin force, the proposed maritime upgrade to Sentinel and the long range SAR (of sorts) provided by the C130J (presumably to be taken on by Atlas?).
I don't think we'll be rushing to reopen Kinloss, St Mawgan and Ballykelly for hoards of Poseidons - more like squeeze them into Waddington or use the 'UAS Superbase' at Wittering, or spare space at Leeming.