The RAF C-17s operate to the Global Fleet concept, which keeps them supposedly in step with the rest of the world. The slight irony in that is that it actually puts us ahead of most of the USAF jets - I have no idea how the 'queue' (or 'line' for our Stateside buddies!) works for the upgrades. The fact is, we get each and every one of them.
I understand that. So do several other nations. But not all nations opt for every upgrade. The Global Fleet concept does not require every aircraft in that fleet to have every upgrade. Indeed, some nations are prohibited from getting some of the upgrades/systems. Further, I attend the semi annual CORT and SORT gatherings of the global C-17 fleet. The UK folks who attended the last one (in March this year) indicated that since they now own their aircraft rather than lease them, they no longer have any operating restrictions and they intend to use every capability. They've already tested some of the troop drop capabilities, are buying equipment compatible with the C-17's equipment drop systems, and will test/qualify that equipment and systems at the earliest opportunity. And the UARSSI system was not fully functional when the aircraft were delivered to the UK, but every UK aircraft that goes thru San Antonio has that system made fully functional. And that has nothing to do with the block upgrade program.