I didn't know that the "low-costs" didn't have seniority lists! Definitely has some advantages as I think they're a double-edged sword i.e. they can save you from demotion/redundancy, but they can also stop you from working for the company of your choice....
What happened with KLM/Air UK - I assume they stayed pretty much separate? Would also be interested to hear about BOAC/BEA and any other seniority list merging.