Easy,
Pretty sure that the USN have already flight tested a WESCAM fit on P-8 as well as a radar under the forward fuselage for GMTI (as shown in your pic). The USN is looking to turn the P-8 into a true replacement for all the specialist versions of the P-3 so there are all sorts of interesting lines of development that the UK can piggy back onto.
CptDesire,
The P-8 has a weapons bay in the aft fuselage for torpedoes and chutes for sonobuoys.
The UK has had some of its team that were testing Nimrod MRA4 heavily involved with the USN testing and intro to service of P-8 for some years.