I was addicted to mentadent way after it was no longer cool to use it.  It's a much better toothpaste, but it just seemed wasteful to have to throw away all that plastic when it was empty.

Try Drugstore.com or whatever the UK equivalent might be.  You may be able to get it shipped to you from the US (assuming it exists in the US).

Otherwise, you can always just skip toothpaste altogether.  It's not neccessary for clean teeth, and if you like the minty part, you can use mouthwash before and/or after.  It's more important to brush lightly (to avoid gum recession) and floss daily.