orbitmic

Besides the obvious, it is not evident to me that you can "swap with a friend" anyway. As cws mentioned, the most likely scenario is that either everyone will fly, or they will ask for volunteers. However, if not, bear in mind that there may be more than one person denied boarding, and if you are willing to give up your seat, however unfair it may seem, it seems to me that the system will have its own priority order which may not necessarily put your friend top of the list. It may well be that the ground agents will allow and manage exactly what you suggest if it becomes necessary, but equally, it may be that they give you a chance to volunteer not to fly, but not to decide who would in our place.

Also, bear in mind that it is perfectly possible that they may look for volunteers to get offloaded from ET but not from CE if CE is not full and Y is overbooked (not an unusual situation per se as with variable cabins, BA will usually prioritise accommodating everyone in C, but the C pax numbers may not always be divisible by 4).

Either way, I'm pretty sure your choice not to fly would be deemed voluntary so if willing to go that route, I think you should clarify with ground staff in advance what would be on offer, notably whether this would include accommodation.