Chaps, the C130J and A400 ideas are nothing more than concepts. Every future project I have seen promises the earth within budget, in reality they rarely, if ever produce the promised capabilities on time or in budget.
The cost isn't just in the airframes; you need a huge support train for a multi-sensor aircraft on top of the existing airframe support contracts. For C130J that would be, maintaining an ageing airframe (with probable legacy issues already) to operate in a low level maritime environment (fatigue and corrosion?), plus this miracle plug in pallet solution which will be massively harder to develop and maintain than the picture of a box on the concept drawing. Don't forget the crews need training and practice so you need mission simulators, as the airframes will hardly ever be available. All for a hugely compromised capabilities which would hardly ever be available, and not at the same time you require AT (like in a conflict perhaps?)
Haven't we learnt the false economy of recycling our airframes, and the massive issues it causes? Swiss army knife solutions normally end up as declared capabilities sat in the corner of a hangar that the engineers and aircrew know don’t work properly if at all.
Off the shelf developed hardware with future potential that is already being operated by others is the only sensible option, the only question is what is our requirement? If it remains high end ASW then P-3 or P-8 are the only sensible solutions.
We will be able to fit 8 – 10 at Waddington when the E-3D is cancelled next year and we join the NATO E-3 pool and forward base them
(now that would save money, where is that GEMS form!)