It makes strategic sense for a Nation wanting to play a part on the world stage and remain at the forefront of NATO operations to have a Tanker capable of refuelling all receiver-capable aircraft. I'm not so convinced about the benefits of also adding a UARRSI to their A330 MRTTs, but it would make for a complete package.
With other nations having proved the operational capability of the aircraft and AAR systems, and with individual crew members having gained significant experience in training and operations, it should be fairly straightforward for the RAF to introduce a boom and UARRSI.
If I understand the Voyager program correctly (which may be doubtful) the airframe needs to maintain its civil certification and be able to be operated as a civil aircraft, flown by civil pilots, on a civil (i.e. passenger carrying) mission. If this is correct, then a UARRSI and other purely military systems becomes problematic. MDC and Boeing both did a lot of work to build and sell civil versions of the C-17, and UARRSI and other purely military pieces of the basic C-17 were a real problem. In the end, no civil C-17s were ever sold or built. And this would have been a freighter, not a passenger carrying airliner.