Just my view (as ex-UK MPA) - a C130J/A400 fit is possible of course but the only 'realistic' (read affordable) option for the UK in this area would be the existing USMC package. Now - how may frames do you convert? Do you buy those frames or lose them from the lifter world? You then also have the 'fleet within a fleet' issues.
Far more 'realistic' (see above) is to just accept that:
1. UK Gov has scraped by without MPA for nearly 5 years now - they need cash in the piggy bank to look good at budget time - not expensive MPA that "we don't really need" (not me saying that - but I suspect the view of 90% of Joe Public?)
2. If really, really, really pushed (RN get a Russian email with a nice pic of the arse end of the deploying V-boat?) - we put in place a lease/buy deal on 5 x P8s (offsetting pretty much all capital costs until after 2020). Also keeps USA happy - we play with them again and we buy their kit. Of course, this means someone has to swallow the pain elsewhere - maybe tied into the deterrent replacement program?