Firstly, are you sure that it's an 'MPA' which is needed?
Surely it would be preferable first to examine which defence capabilities you now consider to be missing - e.g. anti-submarine, long-range surface surveillance, long range SAR, electro-optical reconnaissance, (classified) role etc. Then decide whether these need to be met in a single platform, or in multiple specialist platforms.
No doubt the airship and drone fantasists will attempt to dream up useful applications for their air platforms, but my gut instinct is that airships are a complete and utter waste of time and drones do not yet have anything like the flexibility or load carrying capability of manned aircraft.
Who should operate over the sea? RAF, RN or an enhanced coastguard? Does it really matter, so long as the task needs are met?