If you are persistent you will get the compensation you are entitled to, which is considerable. If BA told you the flight was cancelled but then ran it as BA 106x the following day then that is technically not your problem. They will probably not pay up willingly. Perhaps the best tactic is to just write and tell them what happened and see what they offer. Leave the EU compensation entitlements in your back pocket for now. If you don't like what they offer then ask for the EU compensation. Expect to have to take them to a small claims court to get the money though. I think that buying a ticket on EK was probably not the best tactic; understandable in the circumstances but it you may not get the money for that.