Most of them appear in front of me when I'm late for work! Seriously I think they must get distributed around the country to other operators. I saw an ancient double decker the other day. That was in the Manchester area, but I have seen old ones around in other places too. I guess the people who do the sums have worked out that it is more cost effective to buy and sell them at specific ages/mileages than continue to maintain them. A company with a different cost structure may be able to continue to maintain them for a while, then another takes over and so on. It becomes a balancing act between the initial cost of the bus and the on-going maintenance costs.