A third operational Voyager must remain in the United Kingdom to support fighters flying domestic air-defence missions.
If that means a fully-fuelled Voyager sitting around waiting for a Q launch, that must surely mean airframe fatigue issues? Not many airlines keep an A330 sitting on the ground at MTOM, but presumably the requirement was known to Airbus when the Voyager was specified...???
Whether 3 x A310 with around 72t each (3 x 72 = 216) rather than 2 x A330 with 110 each (2 x 110 = 220) would have been a better choice is a matter of history. But originally the Future Tanker Aircraft was intended to replace the entire VC10/VC10K/TriStar K fleet with around 24 airframes of A310 size.... Rather more hoses in the sky than you see today.