Pingu,
I agree with all you say about the MRA4 & Helix, but not about putting it all in a smaller jet. While the Global Express, as used for the Sentinel R1, has excellent range, it seems that it's far too small for the Helix equipment & number of operators.
The truly ridiculous thing about this is that it's been decided - and publicly announced - that the three MRA4 development airframes will not be turned into production MRA4s. That gives us three spare MRA4 airframes which we could fit the Helix gear into. Since the C-135 purchase would necessitate converting KC-135 airframes, & fitting what is supposedly a non-standard UK-only version of the RC-135 equipment, I can see no advantage to it. The rebuilds will take time & cost money, there are development risks, we have to pay for the airframes, it introduces an additional type into service, with unique engines, the airframes are 40 years old . . . . why not fit the kit (with, admittedly, some development risk still) into mostly new airframes we already own, of a type already (by the time the refit is done) in service, with engines shared with yet another type, & with the few 40 year old parts remaining all zero-lifed . . . . Both acquisition & operating costs should be less.
And, of course, the original Helix plan involved integrating it into the old Nimrod airframe. Development work done so far should need less adaptation to fit into MRA4 than another type.