This has come up before. The explanation is that the UK and Japan are the only countries in the world that have postcodes as precise as seven digits. The general world standard is a much less precise four digits. If a satnav is designed to work to seven digit codes, fine, it has no problem working to less precise four digit codes. But if a system is designed for four digit codes, while it can be reprogrammed to work to seven digit codes it won't always work properly to that degree of precision. I’m currently enjoying excellent service from the wide screen satnav in a BMW 320d, that even routes me round traffic jams.