I can't answer fully but we had a Regional Executive in Montevideo whose name has slipped my memory (he was Bill, not Sandy). The de Havilland Dragon Rapide was used for sightseeing and joyriding flights worldwide, so I'd guess you flew in one of those.
Aerolineas Argentinas had Comet 4s and my guess is that you flew to the UK in one of theirs which would have flown from Buenos Aires Ezeiza.
ETA Barry's post has reminded me that BOAC Comets did fly to Montevideo but I can't say when that would have been.
Last edited by Allan Lupton; 9th Oct 2016 at 08:46.
Reason: extra point