ICBM,
If I may I would argue that the fact the F-35 will replace GR4 actually nullifies any argument that the two need to exist in different airfields. Afew years of squeeze is irrelevant for a fifty year capability.
Am I right that Marham is the site for some GR4 depth activity? In which case let the GR4 die there, then close the base. (If I am wrong please disregard).
I have to disagree about the East coast ranges. Holbeach and Donna offer little to F-35. In all honesty neither do the 323s and 613s. What F-35 needs is a short sea transit followed by meaningful training overland. Not the 'charge up the north sea through the 323s and pickle somewhere north of Newcastle'. That might be meaningful for the (single and a bit role) Typhoon, not our new striker.
We have big MDAs west of Cornwall and north of Lossie - we just hardly ever use them. (Unless we've lost them as a result - I am not UK based at the moment so can't check)
I think Marham is a silly idea, but that is just my opinion and I am more than aware that none of us is paid to agree with the next chap.
Have a good weekend, fly safely.