Why the C-130 beat a unique British HS.681 airlifter

Would procuring the HS.681 have been better than buying C-130s?

No; the unique HS.681 support chain would have been costly, while C-130 commonality made logistics much easier.

The HS.681 must be judged against the support system required to keep an aircraft fleet available as well as its paper performance. A uniquely British type would need its own spares, training, maintenance equipment, and production base. Buying the C-130 placed Britain inside a much larger international logistics network and reduced those burdens. Retaining C-17 production or developing a future replacement collaboratively could offer more value than maintaining an isolated HS.681 fleet. Commonality can outweigh national design ownership in military airlift.