Neither writing "theatre" instead of "theater" or "Hamlet" instead of "c/o Hamlet" or anything like that should have made a difference. It's very unusual for an RTS to come back like this, you have the address correct. It's true that Americans and Brits dont write 1s the same as mainland Europeans, but I would have assumed that the postal system would have spotted the difference given the rest of the address.
By the way (this wont have affected you getting an RTS, it's just an observation) you might want to write your addresses closer to the middle of the envelope, as that's the easiest place for the computers to read it
