Back when Voyager was still FSTA, the RAF was trying to see whether the aircraft (whether B767, A310 or A330) could be operated with a 2-person crew. After several expensive and pointless investigative sessions, the decision was made to use a 2 person crew for AT, 'augmented' by a food-powered pump attendant for AAR. Whereas more sensible nations listened to those with experience and settled on a 3-person crew for their new tanker fleets, so that the pilots' workload wouldn't be increased trying to manage AAR dynamics as there would be an ex-Tornado / F-4E / C-130 navigator in the 3rd seat with a fit-for-purpose AAR mission planning and management system at his/her disposal.
It was even recommended that FSTA should have a boom "If only to guarantee a 3-person crew"! 20 years ago, BAE's A310MRTT was also offered with a boom.
But back then, no-one ever thought that the RAF would have quite so many receivers which require boom AAR - and if a decision to change the F-35 order were to mean half F-35A and half F-35B, unless the UK is stupid enough to sign a blank cheque for UK-bespoke F-35A + probe mods, that number will only increase.