Well this is a bizarre story if ever I've heard one. 

The UK arm of European charter airline Thomas Cook is set to lease a de-militarized Airbus A330 tanker from the AirTanker consortium.

AirTanker, which owns and operates the A330 Voyager Multi-Role Tanker Transports (MRTTs) for the UK Royal Air Force, has signed a letter of intent with Thomas Cook for an initial three-year leasing deal to remove military equipment and install a new interior in the A330. The move will support Thomas Cook’s growing long-haul network.

So even after taking out all the military-grade equipment and bringing it back to passenger spec, it's STILL cheaper than leasing a pax-configured airplane. Says a lot about the current mid-range aircraft market. 

There's also an interesting side-deal as part of this arrangement: 

The deal, announced June 24, will also see 10 pilots seconded from the charter company into AirTanker, which will see Thomas Cook pilots flying AirTanker’s civilian-registered A330 on military charters, such as those to the Falklands or Cyprus.