Passing by Royal Mail website, I discovered this article title: Impact of new EU speed restrictions.
Reading it, I discovered that, since 1st January 2008, all vehicles weighting more than 3.5 tons can't move above 90 km/h inside the European Union.
And there Royal Mail is obliged to explain that, because of the Union, mail might arrived at your post offices or post boxes a couple minutes later than usual.
But, in town, you will see your mail before 2pm (3pm in the country). I don't know how a British postman's work is organized and if he really finish his route at these times. But, in France, I haven't seen a postman in the streets after noon. Perhaps, mail sorting is differently organized: in France, postmen begin early in their local post offices by sorting the mail of their routes.
If this tiny article gives ideas of compared studies of the work of postmen...