Are you saying the RN has a surplus, Bismark? If so, you'd better hope the Chancellor doesn't read this. If not, who will fill their shoes? If the RN has a secret pipeline running with a capability greater than its requirement, watch out for SDSR2015.
No announcements have even been made yet. If decisions have been made, we don't know about them yet.
The biggest problem is that you and I see buying P-8 as re-establishing a long-standing capability. In Government funding terms it is now, effectively, introducing a new capability with all the other expensive stuff that involves. There will have to be trade-offs and the Navy, sadly, is likely to have to give up its share of "other stuff" in order to maintain/secure the funding for the carriers, aircrew for F-35, support issues, existing programmes, etc, etc.
I would love it to be joint, and I hope it may well be, but there is no slack left. I suspect, if it does become an RAF role again, the Air Force will have to give up something else.
Nothing is free these days.