I would have thought the layout of a 4 class BA aircraft is better suited for a charter full of journalists and other guests, so the PM and his aides have the F cabin to themselves to discuss business in private. The BA F seat is more amenable to working than the VS UCS.

Tony Blair in BA F was the image for the front cover of Alastair Campbell's book "The Blair Years". Link

Perhaps Gordon is not a fan of the 777?