The aircraft which Thomas Cook will operate is one of the 5 'surge' fleet aircraft - if I recall correctly, the RAF will operate 9 x Voyagers and the others will, like the TC aircraft, be leased out for third party revenue generation. However, I suspect that one significant benefit of the deal for ATr is the availability of more civilian aircrew to augment those flying their A330.
AW&ST: ...........The deal, announced on June 24, will also see 10 pilots, six captains and four first officers, transition from Thomas Cook to AirTanker. They will then fly AirTanker’s single civilian registered A330 on military charters such as those to the Falkland Islands. By transitioning the crews, AirTanker does not have to recruit many pilots, at least during the course of the three-year deal........