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You can even build in a nice ~24 hour stopover in Lisbon too if you like!
I found that Google Flights seems to dig up the best fares, though you might want to try Momondo (etc) too.
The very lowest fares (like the one pictured above) are with the unfortunately named “Lucky2Go”, which is an online travel agency I haven’t heard of before and it doesn’t seem to get great reviews. You might well be better off paying something more like £630 and booking with a better known agency.
Availability looks pretty good all the way through to the end of 2020:
Depending on which airline loyalty programme you decide to credit the flights to, you should earn between 7,000-12,000 redeemable miles.
If you are wondering about how best to fly home, using points/miles/Avios from Brazil is a fantastic option because ‘fuel surcharges’ are banned there. In particular, you might want to take advantage of being in Brazil to book an epic Asia Miles award on British Airways without any surcharges! For example, Rio to London (stopover for a few months), then fly to Sydney – all in Business Class, for a total of 90,000 Asia Miles/Amex Membership Rewards Points…