There's something extremely cool about the feeling of getting two for one, let alone three for one. That's especially true when the two or three, are the best cities on the East Coast, as well as the best cities in South America; all for less than the price one usually pays to either, not both. There's an incredible fare departing London floating around, which allows you two or three vacations in total business class style, all for the price of one...
American and British Airways have a very intriguing fare, which for once actually starts in London. In the fare, you're allowed an extended stopover on the East Coast USA, allowing you to potentially see two US cities on the way to and from South America, the final destination. In South America, you have the tough choice of visiting Rio, Sao Paulo or Buenos Aires. Perhaps with some magic you could even open jaw and see two of the three. My vision is to leave London, arrive in New York for a few days, head down to South America for a few days, come back to the US via Miami, stay in Miami for a few days and then back to London...
Wide dates from April 2016 all the way to February of next year, 2017, and possibly beyond, including peak holiday season. I just successfully priced a pre Christmas trip, with extended stopovers in New York, returning to London on New Year's eve.
This can be booked directly on American Airlines, at their website AA.com. I personally prefer using Google Flights, which is simpler and more efficient. When booking on the site, you'll want to use the multi stop tool to try and build the itinerary, forcing the stopovers you would like. The key is to enter LHR to EZE, GIG as the first leg, and then for the return home EZE or GIG to AMS, with a connection in London on the way home. If you struggle to spit out the price and forced stopover options you'd like, I highly recommend Adam Corin at FlightCentre, who just built me a custom itinerary forcing the stops I'm interested in; allowing me to break up the fare over a month. Adam.Corin@flightcentre.co.uk is the best way to get in touch.
Here's one I priced out, as an example: Google Flights
Use a travel agent to force the stopovers you want. I live in New York, so for me, I want to come home, get to work for a couple weeks, go to South America for a quick trip, get back to work for a month, and then head to London. It's possible. If you're into collecting miles or tier points, you could easily earn over 600 British Airways tier points with no effort, that's instant Silver status for the next year. Whichever airline you fly, you can earn either American or British Airways miles, gaining you a serious stack, and some status!