I nonreved on BA about two years ago, from PHX to LHR. You have to list on the above mentioned website I believe 48 or 24 hours prior to departure. They were wonderful, flight was open so they released our seats at check in 4 + hours before departure. At LHR I believe they have a non-revenue check in desk were staff checks in. As a side note you must purchase the ZED pass before you list for your flight.

Regards.

Chepos