If it says region 1 somewhere on the DVD box ( usually towards the bottom at the back of the cover ), it is an American disc.
If there is no BBFC ( British Board of Film Classification )certificate on the cover, then it isn't a British release.
It should state somewhere on the web page where you bought it on Amazon, what region it is.
Check the Internet for the code to change the region on your DVD player ( there may be a hidden menu ).
Do a Google search for "DVD play hack", and search the make of your DVD player.