If, and it's a very big if, this does go ahead there are many issues to discuss, no doubt most of which will soon be raised by ex-maritime aircrew. However, just to start the ball rolling until they chip in:
5 aircraft won't be enough to fulfill any overseas deployments as well as providing essential UK coverage - given that no doubt at least 1 will always be in long term maintenence. No doubt a "we'll deploy 1 on exercise, but it is always subject to short notice recall" policy will be adopted so they can show their face in some parts of the world.
They won't be based in Scotland.
The P-8 and MRA4 don't/didn't have the same capabilities (a thread on its own no doubt).
Given the current shortage in the Defence budget, with extra cuts looming, what goes in order to be able to afford this extra expenditure? Or is this yet another, "when the economy picks up in 2015 and the Defence budget increases again" aspiration that will probably never be fulfilled?
If we ordered them tomorrow (see point above) they probably wouldn't arrive for at least 3-5 years.
Who do you plan on manning them, given that many of your ex-maritime aircrew will have left or moved on to other fleets by then? In addition, who would volunteer for, or perhaps more importantly chose to remain long term and gather experience on, a fleet that they know has already been scrapped once when money got tight.
etc
etc
etc
The list goes on and on!