Marham is a good choice in comparison to others that are available and viable. It is a short hop to the east coast range complex which runs pretty much from Norfolk to Leuchars. It is a well-established base which will be close to the USAF F-35As out of RAF Lakenheath affording a close-knit daily training relationship with a common platform. Since much of F-35's threat training will be simulated on the avionics the relevance of needing emitters at Loch Ewe, Cape Wrath and such like is not persuasive and the issue of congested airspace to the south of Norfolk can be worked around - go North.
Would St Mawgan/Yeovilton/Lossiemouth be better? From an airspace perspective probably yes, but then you'd have to transit a fair way to work with USAF F-35As and for the southern options, Typhoon (or they'd have to transit to you). Bearing in mind that the RAF's F-35s WILL be the natural replacement for Tornado GR4, it makes absolute sense to preserve RAF Marham given the lack of significant counter arguments and constituency politics concerning local jobs.
I guess we await the formal basing decision in the meantime.