Seat allocation and check in for code shares
I'm thinking of going to Chile. If I go to the LAN Chile website one of the options it offers is LHR/MAD using a LA flight number operated by BA and then MAD/SCL using a LA flight number operated by IB. Assume for a moment I book it - which web site do I use to allocate my seats? LA, BA or IB or a combination? If I can use the LA website to allocate both seats and given that BA now charge for seat allocation does that mean I'm avoiding the BA fee?
Then we move on to check in. If I ignore the online option then I'll go to the BA desk at LHR and (hopefully) they will give me boarding passes for both flights. If I want to check in online which website do I go to? I would like to think LA but I'm doubtful.
Anyone know the answers?