One point that has been brought up a couple of times already, but does not seem to have registered, is that the RAF RJs are unique aircraft, not merely more USAF RJs with different roundels.
All the USAF RJs were originally built as EC135B, and have served their 40+ year careers in the relatively benign environment of communications platforms or intelligence gathering.
The RAF RJs are converted from surplus KC135A (latterly KC135R) which will have led a very different operational career. I am sure that L3 and Boeing have taken the different stresses involved in AAR into consideration, as well as the complications or removing the boom and associated plumbing.
These changes will surely have generated a mountain of unique paperwork which will presumably have to be (or have been) checked and verified by the UK authorities.
Summarizing: The RAF RJs are NOT merely more USAF RJs, so please don't use that argument.