As other have said it wasn't imported from the US to the UK about 20 or so years ago. Actually I'd say that here in the Scottish Borders it is less of a to do now that in was when I was bairn. We called it 'guising' and you got dressed up and went from door to door doing songs or telling jokes in return for treats. We also dooked for apples in bowls of water and toffee apples were hung from the ceiling for you to eat without using your hands. I think the change from calling it "guising" to "trick or treating" happened after the film ET. Even here they call the actual guising 'trick or treating' now but they still actually mostly do the jokes or songs too. A change in name for the most part. Kids still dook for apples but the toffee apples seems to be a thing of the past now - at least here. Fair do in that the parts of the UK which had no real Halloween tradition (would that be more parts of England and Wales?) may think the tradition is American.