I almost posted a while ago to ask about something that I had heard from an extremely unreliable source who is most certainly not BA staff and not a regular flyer but claimed to have a friend who was a pilot. The person in question simply told the staff member which flight he was on, they emailed 'an address' and after boarding the CSD came and moved the person from WT to CW (some kind of space-available, on-board U/G).
I wouldn't have believed anything about the story except I did see both a photo of this person sitting in a CW seat and their WT BP (not the ones referred to above).