I remember seeing that Chieftain tank disguised as an M1 in the 'The Walking Dead'. I was surprised and it made me think initially that the series was made in England considering the lead actor is also English.
But I think it's something to do with the some law where you can't get an M1 surplus anymore even for a movie. Considering you can buy any number of British and Russian MBTs that's seems a bit odd.
The only real tank I ever saw was at a paintball venue here in Ireland. It was a Chieftain too. It was used in the movie 'Reign of Fire'. Funny enough the Irish army has no tanks. So the owner outguns the whole army