Alright, thank you for everyone's help, you've convinced me - it's not worth the risk. I'll avoid BA and either fly via FRA or pay more for a direct ARN-LHR flight on SAS which, being *A, should be able to through-check my bags to Avianca. (Right?)
I don't know about SAS' policies, but by sticking to LHR T2 you will certainly make this trip a lot easier on a lot of points.