Are we sure that perhaps NO EU carrier bid for the work at all?
I am curious how the US crew could be the cheapest, when they are not UK-based, presumably fly an older aircraft, away from maintenance base, and suffer the same or similar other costs (fuel, crew) as other bidders.
Perhaps the contract is so completely ridiculous, that penalties for non-delivery of service due to technical problems (of an older, charter aircraft without the maintenance base support of a UK-based airline) are so low that it does not matter...
Is it that hard to place an order for a cargo/passenger hybird aircraft, maintained by BA/Virgin, at a cost far lower than the aircraft flown/chartered at the moment. (of course the answer is yes...)