I do not understand the modern RAF pay scales and I have no experience of the Voyager. However I have easily googled RAF/MOD pages that detail both subjects.
One indicates that in pax mode the Voyager has two front crew and eight rear crew.
Another tabulates service pay for 2012. Not being sure how representative this would be, I judge from that table that the front crew might be at level 33 on the Professional Aviator Pay Spine and that the rear crew might typically be at level 16. Calculating on each crewman working a 200-day year, that suggests over £3000 per day just to man the aircraft.
To our resident experts does that seem realistic ? I did see the hint that the aircraft may be flown by civilian crews for some of their lives.
Still, working with my figures: if Downing Street plans a day trip to Berlin and back that might not take all of two hours. It will take all of the remaining £1000 to pay for fuel, ground servicing and the PPI rent of the aircraft – and then some.