Tiger ll Perhaps? One of the few remaining examples is kept in Bovington.
Still not British or American
Maus Tank Maybe? Only one non running example remains period, and it's in Russia I believe, also one of Hitler's dreamed super tanks... You would think he was trying to compensate for something....
Or maybe one of the itereations of the Stalin Tank?
Tell me where your friend saw one of these tanks? What muesum and maybe I can help you find your answer, as in many cases it could be a case of mistaken Identity. I have seen people mislabel a Jag Panther at the Patton muesum before an American tank, simply because it was in America, in all actually it was
Captured by the U.S., not built by it.
Thing is, although there maybe large peices of hardware out there like maybe the M 109, which is a self propelled artillery piece.
The "big" or "heavy" tanks like these died out for a reason, take the Abrams or Challenger ll for example, which is the modern British and American MBT's.
Compared to these Behemoths, is that they lacked mobility and were difficult to delpoy, and often underpowered, which could severely limit their effectiveness and ability to respond to threats, as in the case of the Tiger ll, which if it wasn't swarmed by a unit of T 34's.
Allied Airpower or artillery could have made short work of them once spotted.
The thing is, although many modern armored battle tanks may have slightly different speed performance, but the priciple is the same in terms of mobility.
If a large hulking tank can't cross obsticles, or bridges or what not, it's not going to be very effective on a fast paced mechnized battlefield.