This order was a long time coming. BA could have waited and may yet still order A350-1000's, but that likelihood diminishes with this new 777-300ER order.
I disagree entirely. The official blurb categorically states this is
interim lift, which to me translates into non-cash compensation for the B787 delays in the way of heavily discounted B773s.
Boeing saves face, retains the B787 order, adds a blue-chip carrier to the B773 portfolio, BA gets needed capacity quickly at a knock-down price, allowing accelerated retirement of less efficient B744s; everyone's a winner.
I can't see any reason why BA wouldn't go for A350-1000s when the time comes to issue the RFP for that part of the fleet. 6 B773s and 4 options in no way fulfills that requirement.