Personally I don't really see a major issue in someone opting for another publication other than its being somewhat unfair on the first publication. I guess it's the code of honour, however it's not as if it's a job application. Employers are often fine with a job seeker turning down a role for something else, if that's the given reason. So I don't think an unpaid magazine publication should really be all that different. Maybe I'm missing something? I'm sure BP would be understanding, but it doesn't matter as based on proper conduct I'll sign their contract.