[According to Wikipedia] the aim is to have one post-code per fifteen addresses.

Which means that unlike (5 digit) ZIP codes and German Postal codes, UK Postcodes are typically pretty useful for navigation - people will often enter a postcode into a Sat-Nav to reach a home or business. However, sometimes this can entirely fail - e.g. for a historic house on a large estate which may have access some hundreds of meters away from the postcode indicated location, and so sometimes you'll see a separate postcode to put into your Sat-Nav !

Even more so. Some buildings I lived in had several postcodes. (e.g. some apartments in the building had one postcode assigned, and others had a different one)

In contrast, the 0872 postcode in Australia covers over a million square kilometers.