OK, you're right then. Have it your way but let me say the following first:
1) If you didn't buy your ticket from BA but from an agent then how is it BAs fault if the agent didn't warn you?
2) Information is passed on by telesales agents at BA call centres as it is the law, they have to do it.
3) It is fairly well known that British Airways along with most other carriers posts service information on their website, if you're too lazy/thick/ignorant to bother to look at it or ask questions then sorry, you're gonna get a suprise when you shell out that whole 3 Euro for a bottle of water when you were told by BA telesales that you were flying with Iberia (if it was a travel agent then that's their call)
4) I'm not defending codesharing -- for me at work it's a pain in the ar$e when I'm trying to answer queries about which terminal which flight goes from/ arrives at. To me it seems like bending the truth to say an airline flies to a certain destination purely on the merit of having stuck their flight number on the board in the departure lounge. Why not just sell the flight on the operators flight number (apart from creating problems with the US and other government's suppliers regulations)