Avianca allows pets in cabin on domestic (Colombia) and Latin American flights, not on international flights beyond Latin America or so they say online. You'll have to call and double check.
This site could be useful to you:
http://www.pettravel.com/airline_rules.cfm
There is a drop down menu where you choose each airline individually. Avianca is the only Columbian airline I know. You might want to check other airlines that may fly BA to Medellin.
Then you can check their website. I found this PDF that has the rules. But haven't a clue how old this document is. Let us know if you find anything else.
This site could be useful to you:
http://www.pettravel.com/airline_rules.cfm
There is a drop down menu where you choose each airline individually. Avianca is the only Columbian airline I know. You might want to check other airlines that may fly BA to Medellin.
Then you can check their website. I found this PDF that has the rules. But haven't a clue how old this document is. Let us know if you find anything else.