Miles Magister,
So I guess that you have never been to Cambridge/Teversham? The facility is huge with many MANY hangars as well as workshops, assembly facilities, design office etc etc . etc. Seeing as they are going to be converting Boeing737NG airframes into E-7 aircraft for the RAF that alone will need a LOT of space. Add that to their C-130 work, airliner work, business jet work, manufacturing operations and MRO activity and they need a big area, and that's before you take into account the automotive side.
As for your suggestion about the "big hangar at St.Athan," I guess you missed the fact that it is now a large Aston Martin factory?
I really cannot see Marshall relocating to Cranfield or Duxford, there is no room at one and no infrastructure whatsoever at the other. Even Wyton will need extensive new build and renovation. They can only use the 84 year old C-hangars and airfield, everything else will have to be built from scratch.