For those who are not familiar with the concept of priority codes they exist so that BA can determine which staff travellers should be accommodated in case there are more staff travellers than unsold/available seats on the plane.

In some cases BA may consider it more important to onload staff travellers than regular fare paying passengers. And when flights are overbooked it may not always be the case that staff are offloaded in favour of revenue passengers. For example a positioning flight/cabin crew member may be unloaded when a revenue passenger is offloaded so that they can operate downroute.

A member of staff travelling on a business trip will almost surely have a higher priority code than one travelling on leisure, and there may be variation of priority by seniority.

For example I would expect Willie Walsh to take the last seat in F over a regular executive, a regular executive to take precedence over a manager and so on.

The concept of cabin priority works exactly the same as priority for onloading, but refers to which passengers should be seated first in a higher cabin in case there are not enough comfy seats to go round.