To me, this is very similar to the recent post about email address validation. Very often, companies feel the need to validate certain inputs themselves and do a terrible job, when there's a better solution.

When it comes to addresses, if they're in the U.S. there is an easy way to validate delivery addresses - the USPS. They offer free access to an API that includes address validation, and thankfully most shipping services now use it. Some services still screw things up, but its less of a problem than it used to be. I notice that the Royal Mail equivalent is paid. Perhaps that's part of the problem, or perhaps the Royal Mail validator fails addresses with slashes in it.

I also noticed that the Royal Mail is a publicly traded company. I'll uh, refrain from commenting on that.