If you want an equivalent sized militarized airframe I'd suggest talking to Boeing about a 767 (KC-767) or 737 (P-8A) frame or, as Beagle suggest, Airbus
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Exactly what i suggested (the P-8A anyway).
The B737-900 variant would seem plausable in terms of size and endurance is pretty good too (it has to be for the USN to choose it for the P-8 MARPAT) and with extended endurance via IFR it could make a decent platform.
But we are talking about the UK Government here and they go backwards at the best of times so we are looking for aircraft that are on their last legs from former airlines probably.
Airbus would be good for commonality with the Air Tanker fleet, but hang on... we have to get the Air Tankers first and how much of a wait will that be? And even those are 'leased' and privately financed.
What's next? Privately financing our ELINT/SIGINT platforms?
What about converting two or three of the E-3Ds to ELINT and keeping the spare bits for the rest of the fleet, still leaves four or five. Then get something smaller (Global Express for commonality?) to complement it for the jobs that don't require the whole capability.
I don't think we have 8 airframes available do we? One is in deep maintenance due to a prang from high winds... one or two deployed in theatres, one or so always off to the US for Exercises and then that leaves two or three available for UK exercises and support (including Europe)...
The B707 airframes are just as bad i assume than those of the Nimrods.
The oldest KC-135 in the USAF today is what, 49yrs old or so?