It's not part of the RAF tanker fleet, it's one of the "Surge" aircraft in the Air Tanker inventory that has been returned from lease from an airline that went out of business. It needed an interior refit and repaint anyway so why not assign it to the government as a VIP transport? It's not as if that was a new idea. We had a whole fleet of aircraft and two complete RAF Stations dedicated to Royal/VIP transportation when I served back in the 60s/70s. For long-haul overseas diplomacy we'd happily convert two VC10s for Royal or Prime Ministerial trips at the drop of a hat. But of course RAF airmens' labour and paint supplies are free of charge, aren't they?
