How best can I repatriate money from Brazil to UK? (Rio de Janeiro)

Hi,
I will shortly be moving to Rio to work for 2 years on secondment with my company. I will be earning in Reals during this time and, assuming I save something (perhaps unlikely!), I will want to repatriate money back to Sterling. Does anyone know the cheapest way to do this? I have heard of the existence of "dual currency" bank accounts but the international banks I have spoken to in London don't seem to offer this for Brazil. I want to avoid paying high currency conversion costs if I am transferring large sums back. Any advice from others who have experienced this much appreciated! Muito obrigada!