SR71,
I believe BA plan on a "wastage rate," as they so nicely put it, of 1% - that is people leaving for reasons other than retirement - loss of licence, snuffing it etc etc
I have a "forecast seniority list" produced about 9 or 10 years ago and it is remarkably accurate. (I think it was based on normal retirements plus 1% wastage). I am about 30 places "better off" than it predicted - this year the prediction is closer to my actual number than ever, I was about 90 places better off 4 or 5 years ago - I guess that's because those that left early for whatever reason would now have retired anyway.
So the figures suggest that in the last 10 years only 3 people/year have left of their own accord to do something else.