JiV has the size of it. The issue will be budget driven rather than cost driven. How are the transport aircraft and sentinel funded? Does the Army fund part of these forces?
Logically, for a maritime role, the aircraft would be funded from the RN budget. If the RN can't fund, and own it, and the Air Force wants to return to an historic maritime role, then the Air Force must find the budget for a P8.
They cannot man it with naval ratings unless they pay the Navy for them or the Navy agrees to supply and pay.
They will not fit, say Army mission equipment, unless the Army is prepared to pay.
To get an RAF platform then the maritime mafia has to get the funds from the FJ mafia.