I suspect it was actually BOAC. Here's their Comet schedule through Montevideo from 1961, in your timeframe
http://www.timetableimages.com/ttima...8/ba618-15.jpg
The route was run by BOAC Comets for several years until British United got their first VC-10s, when it was transferred.
BOAC Comet 4s had wing tanks. It was only the medium-haul Comet 4Bs for BEA, Olympic, etc, that did not. BOAC's livery was blue and white, with a lot of unpainted silver, and BOAC written on the tanks, which also fits your description.
https://www.google.co.uk/search?q=co...bqCWre7TkjM%3A
Montevideo and Uruguay were a significant trading centre with Britain (as you were doubtless aware), but service there from Europe seems to have faded away.