I was not proposing developing a new MPA, just taking the content that works...if any, and placing it in a cheap commercial airliner as an interim solution. There must be something that works after £4bn.
First, you have to adapt the commercial airliner. That is an expensive process, and takes quite a long time. This process has been done before: the results are called P-3, Nimrod, & P-8.
I repeat: it won't happen. It is not affordable in the current budget, & would take far too long for an interim capability.
Fitting the already bought sensors to an off the shelf MPA such as ATR-72 or C-295 would cost a small fraction of the price of adapting an airliner & be much quicker, because the MPA airframe is already adapted, already has physical modifications, electrical systems which can power sensors, terminals, etc., designed, built, tested & certified. You save all that work & expense. You also get a much cheaper airframe to buy and operate, with manufacturer support available, & with a resale value if you decide to replace it with a high-spec MPA such as P-8. A bodged-up unique airliner adaption has no resale value, & no manufacturer support for you modifications. I can't see a single argument in its favour.
Alexsa: I'm not at all sure about the wisdom of fitting the kit to used P-3s as an interim solution. Firstly, old P-3s would probably need a thorough overhaul of the airframe, engines, & systems, with a lot of replacements & re-lifing. That would take time & cost money. It's worthwhile if you're intending to operate them long term (cf Spain & the Spanish-modified P-3s for Brazil), but not for something you want quickly, cheaply, & for a relatively short time. Secondly, the operating cost would be much higher than the smaller competitors..