Orvilles Dad,
To understand seniority at BA you have to understand the system. When you are recruited into BA you are recruited on the basis that you are capable of being a BA Captain. Therefore every First Officer is deemed to be entitled to a 'crack' at command training at an appropriate point. When you number is up you get to go and have a go whether you the best or not. Sure some people have been taken aside and it has been 'suggested' they might not want to bid for a command just yet. If you don't reach the required standard on your command training you will then have a choice whether you attempt it again or wait before trying again. The added pressure is that if you fail twice you will be CAT C'd which means not eligible to bid for another command.
So everyone gets a shot, and the actual command training is used to weed out those that are unsuitable which in theory are very few due to the fact that initial recruitment should attract a high caliber candidate. Once you dump this system in favour of merit based promotion the clear career path goes out the window and suspicions always arise that people are doing back room deals.