"Why not go the whole hog and base them as an integral part of the Lakenheath wing including USAF tanker support?" - Why not base an RAF F-35A wing very close? Mildenhall is going to be very quiet (i.e. shut...) soon and Honington is almost as close with a couple of HAS sites. The Rocks would love digging holes somewhere new...
I'm not sure, but is there confirmation of the make up of the 138 ac buy? Just thinking the 35B numbers could get capped at the 60-70 mark to give a reinforced CAG plus spares and the other 70-odd as 35As would make a reasonable sized wing for the RAF. If you shifted the balance further to the F35A (say 40/100) then you could conceive of giving the RN the B in toto and leaving the RAF with the A. Mind you, would the rump FAA struggle to man two Sqns plus dedicated STOVL training unit and still supply the required number of Staff and support posts?