https://www.agi.org.uk/wp-content/uploads/2020/11/BS7666Guid...
# Standard data types used in BS 7666
CharacterString: a sequence of alphanumeric characters
...
If I had to guess, alphanumeric is interpreted as [0-9a-z].The sign printer probably expects this format when printing signs for the government, or worse, has a contract that says the government must provide this standard format for the sign information.
So it's just a government mandated database schema... I don't think that's any better of a reasoning though lol