I just looked up this page for Humax setup and you're right.
http://www.radioandtelly.co.uk/humaxfoxsat.html
Don't know why that should be necessary.  If it rejected your incorrect one it must have a stored database I presume.
If you intend using it out here, I'd just put in a valid UK postcode and then look at what it says on the page at the link about setting up other non Freesat channels.  There's no way that the box can know where it's physically located.