There were DH 89 (Rapides) and DH90 (Dragonflies) registered in Uruguay, so there's every chance it was one of those. Here is a link to their civil register:
Civil Aircraft Register - Uruguay
I think BOAC did fly to Montevideo for a while, though I don't know the details. Usually you would have to fly to Buenos Aires and take a flight north from there.
I can't help with the agent, I'm afraid. I worked for BAe in the 80s in that part of the world, and the agent had changed by then. I have checked my information from around the time, but no clues there - sorry.