Hi esreverlluf
If World Traveller is full then airline staff will get upgraded to the next cabin if space is available, commercial pax will not get upgraded. So if World Traveller and World Traveller + are full, then you will get a club world seat and so on. Working for another airline makes no difference as to whether you get onloaded if there is space available (so long as you are dressed smartly for club and first), the only factor to take into account is that you will be on a lower priority compared to any BA staff member.
hope this helps
Angryfool