Given the proximity to Seattle, may be it was a Boeing aircraft with a Lear jet equipped with camera equipment doing a photoshoot. Boeing used to contract a specially equipped Lear from Clay Lacy down at Van Nuys. Photography was done around the Seattle/Cascades region and an old SAC/USAF base up near the Canadian border.
Just seen one of Air Canada's new 777s here at LHR - it's very difficult to see any insignia. may be what you saw was a new AC B777 having its piccy taken?