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As an ex postman......
In general the problem does not lie in Britain. Most countries seem to have worse systems than ours. Some only deliver once a week, or deliver to an area/address when they've got enough mail to warrant a visit. Here, although the second delivery has mostly been done away with, every address with mail is visited at least once.
Over recent years the routes, which should really be left to the postmen to sort, as they're the ones who know which is quickest route, have been taken over by faceless people in offices playing on computers.
I know it doesn't help, or make anyone feel any better when they've lost mail, but it is a fact :-)
Well it was about 18 months ago, things may have changed since then.
Winnie;-)