The history doesn't back the C-130 theory, I'm afraid. As bvcu says, the RAF's C-130s were ordered in 1965, once the HS681 project was cancelled. The Belfast order for 10 aircraft was placed in 1960 and, of course, there was no airdrop requirement as part of that. Lots of this was covered in a Belfast thread in History/Nostalgia earlier this year.
For research on the Belfast, I could recommend C.H.Barnes' "Shorts Aircraft since 1900" which covers development and eventual disappointment at lack of commercial sales at some length.