LJ,
It's not that it couldn't be done. It just couldn't be done in a realistic timescale at a realistic cost.
Ignoring the Arthur C Clarke view of the future, the reality is that in the early 80's UK plc tried to design and build it's own AEW ac. It happened to be called Nimrod AEW and although the airframe worked (ugly as it was), we the UK failed completely to achieve the mission and the ac was scrapped. Into the 90's, our biggest defence contractor was tasked with giving the MR2 a lick of paint, new wings and engines and a bit of an upgrade. 15 years later, they/we/UK plc had failed again. Do you really think we could do what your asking on our own. If not, and we tried some European consortium approach, then how long to IOC? RP suggested 20 years. I think nearer to 35!
So developing an entire new system that can operate at 1,000nm with 6hrs on task and the ability to carry 200 sonobuoys and 9 torpedoes that is fully networked linked would be challenging to say the least. Granted, there would be potential to make the add ons a bit smaller but how much smaller is a 500lb bomb nowadays compared to 1940?
We don't just need a real airborne ASW capability now (and ASuW / LR SAR), we have needed one continously since aircraft were invented. That leaves the option of P8 (off the shelf). CN295 (80% performance/30% cost and off the shelf) or leasing/buying old P3's. Then maybe we could start talks with the French in 2020 to look at some form of UAV in about 2050!