First off ECHS was rejected as it was too expensive (and at time untested). There was going to be a version of Skydel CHS so that the J could carry and drop the existing inventory of UK heavy drop platforms. This was built and test fitted but Lockheed wouldn't back its fitting and use so it was dropped. Too late to retrofit the ECHS the UK went for what the (deluded) movers wanted all along - Dash 4a. When the 4a beams arrived for fitting at Lyneham the holes in the beams didn't line up with the holes in the aircraft so the beams were filed out to fit, thereby ruining them as they are classed as primary structure. The reason the holes didn't line up was simple. Being the lead customer for the J the UKs aircraft were built using the jigs that had built every other Herc and these jigs were worn out (Part of the offset from the UK buy of the J was the supply of new jigs, but these didn't get to Marrietta until our 25 aircraft had been built).
So that is how the RAF J Hercs have the heap of rubbish that Dash 4a is instead of a tarted up version of Skydel.
Before anyone steps in and says the 4a system is modern and Skydel isn't I'll have to be forced to dig out the NATO report that called for a cargo restraint rail system. The Yanks went for Dash 4 and the UK went for Skydel that fitted its fleet of Transport aircraft, the VC10, Argosey, Andover and Hercules (the reason the K floor is different to any other Herc).