You can use Avios assuming there is availability but it looks like you need to book the legs separately. In total it’s 14500 Avios and about £25.

https:/…reward-flights - click on Reward Flight Calculator

If you want this to be booked as a single ticket you may need to give BA a call and ask them once you have found the flights you want.

Flight in South America is expensive. I have travelled to the region 7 times now and I found that internal flight in the region is expensive. The problem is not many competition in the area. There are only LATAM (merger between LAN and TAM), Avianca and Aerolinas Argentina for major airlines. There are smaller airlines like Sky or Gol but still they are also expensive. Again, it’s about less competition in the area. I’m not sure booking early would help. I usually travelled to the region in Nov or christmas time and I booked flights in Jan but the price difference is negligible.

You probably can use Avios on LATAM but I doubt the availability is there if you try to book regional flight. If you fly within Chile, might be worth looking at LATAM pass. Also, what I found is travelling one way is really expensive even for short haul so if you can, try to book return trip.

Good luck on your planning. I might be biased since I love South America but the cost will worth it.

Try booking your internal flights with your international flights.

I am seeing LON - LIM / CUZ - BUE - LON for £900 or less.

Like I put on your post in the Peru forum, international flights within South America are expensive because there's little competition.

I would agree with post 4, see if you can book the entire itinerary as a multi city itinerary. Sometimes that is cheaper than booking the middle leg separately.

I have found LATAM to be about the best way to use Avios, since there are none of the ridiculous surcharges that BA imposes. Due to some glitch, I have even earned Avios on reward flights. That said, I agree that the OP should consider booking the entire trip ex LHR on one ticket.

Hi - My complete itinerary is LGW-Lima-Cusco-Lima-Buenos Aires-Iguazu-Rio-LHR

I am planning on using my BA 2 for 1 voucher from LGW-Lima and Rio-LHR so there may be some issues booking my internal flights with my international flight.

Yes, if you mean it's companion voucher, you can't do it as it's only valid for flight operated by BA.

Indeed, on a 2-4-1 voucher you will not be able to add on non-BA operated flights. You can still use Avios but you will need to take care that the connections give you enough time on separate tickets.