restricting shipping methods based on customer location

on my site i have shipping modules set up for US (media and priority mail). Cananda/Mexico (air mail) and all other international countries (international air mail).

when a customer checks out they are getting shown all these options and many people from the UK are choosing the much cheaper US rate. How do I make it so that the orders from specific zones only see the shipping that applies to their zone?

I was told this, but am not sure what it means:
"The issue of restricting shipping methods based on customer location can be done by creating customized shipping zones and linking those zones to specific shipping modules. This is not an 'out of the box' feature of Zen Cart."

any help is much appreciated

Connor Kerr in Ireland developed a comprehensive Regional Tables shipping module that will do what you want. It's quite detailed, and takes a bit of time to figure out, but it is excellent. see here