the proverbial Knife in the back
It may be proverbial but it is certainly not real. It is probably just another feeble agenda driven attempt to drive a splinter (they can't find a wedge) in the UK/USA relationship.
If any of this become fact, they would have a right to be pissed. At this point it is about as significant as the editorial criticizing the US military for not playing soccer and having tea with the Iraqi civilians.