The simplest way to prevent anyone using any address other than a UK address would be to go into the countries list and delete every country you want to refuse sales to.
In the upcoming v2.0 release you'll be able to mark individual countries as disabled instead of having to "delete" them. But in the v1.x versions you'll have to delete them if you want to refuse to do business with them.
MODERATOR NOTE: Your duplicate posts have been combined. In the future, please post only one new thread for a topic ... multiple duplicates creates confusion and disjointed answers, making it hard for people to find answers by using the search feature ... and I recommend that you use the search feature when preparing to post a question in case it's already been answered.*