I agree that the C-130J that the US Coast Guard is flying is pretty close to a "Sea Herk" MPA, minus the weapons of course.
The operator stations are on pallets and are are well forward in the cargo bay allowing plenty of space for still hauling trash- or the pallets can be removed. Plug and play. Plenty of cargo capabililty remains.
USCG have also been testing bulged windows on the side doors for the Mk.1 eyeball optical system. The bigger fuselage windows on earlier HH-130's are a bit missed.
And we now have the Harvest Hawk KC-130J with weapons fit, and room for expansion- and the ability to pass gas to other aircraft, including helos.
Not sure the UK would have to "buy" more airframes with this idea in the LockMart proposal. Some UK J's will be slated for retirement in the early 2020's when the A400 come on in strength- this could be a conversion.
I do think a demonstrator with the the best of the Coast Guard and Marine equipment, including the drouges, would be quite attractive. Still a MPA though, not a true MMA- that is a a bigger leap and more money with all the additonal sensors, operator stations, weapons, MAD etc. I agree that would likely result in a more dedicated MMA Herk.
Unsure if you would really need an isolated, sound proofed "box" or trailer to do MMA work in a HERC. Plenty of MPA, ASW, ELINT, SIGINT and other work have been done in some quite noisy aircraft. Say again over???
P-8 is attractive as it is closer to MMA. If you can only afford one, there might be desire to go higher end.