Dog Breed?

I am looking for a specific breed of dog that was used to guard scotland yard, are VERY territorial, and are very rare. I watched something on a discovery channel about them coming to the united states about a year ago. They are very large and furry. 100lbs plus! I know what they are NOT mastiffs, danes, shepards, etc. so please do not respond with answers of breeds that are VERY common!! Thanks! They are also very powerful animals that are very hard to tame, almost like they have a behavioral problem