It's interesting how the debate gets framed in the Torygraph, e.g with reference to a "worn out" fleets of E-3D's. Given these were brand spanking new in the early 90's and the USAF (and ourselves) are happily flying '135's that are a good 25-30yrs older, and that they've hardly been flogged to death then I'd describe them as anything but worn out!
That said, doesnt necessarily mean it isn't, or couldnt be made to appear, cheaper to replace than refurb. I can see the operating economics of a combined P-8/E-7 fleet, but can't quite get my head round how purchasing a new fleet of (say 6) expensive high tech aircraft is cheaper than a MLU on E-3D? Assuming that E-7 was based at Waddington you've then got setup costs there, with no doubt quite a lot of new E-7 specific support/maintenance kit + training.
Call me a cynic, but I suspect the Boeing salesmen know full well that the headline costs of E-7 purchase vs E-3D MLU only tell part of the story and that actual costs to get E-7 into service and supportable will be upto a 100% uplift.