If you book a ticket through British Airways, online or over the phone and choose a codeshare flight you will be informed. That's the law. Whether you heed that notification is your choice. If you then choose to assume that every airline has the same onboard service then you are bound to come back down to earth with a bump.
In flight service and what you can expect in the eyes of the law... British Airways was using EAL and one of their old 747-200s (co-incidentally ex-BA anyway) during crew shortages to operate LGW-TPA-LGW. BA were taken to court over this by a group of customers who thought they had not received what they had paid for. It was ruled that what they had paid for was transportation from London to Tampa and back, the inflight amenities were not part of the airline's contract with them. You may also recall the ex-AML 777s that had the "extra seat" in economy -- 10 abreast instead of 9. British Airways undertook to ensure 100% disclosure of this to it's passengers not because of any law that governed the provision of inflight services and amenities but as a
PR exercise.
Seamless service seems to be dependant on your expectations. What you seem to be after is "identical service" -- with the best will in the world, it ain't gonna happen. The service that BA and it's partners provides is referred to as seamless because you can approach any employee of a member airline for service on behalf of any other airline member. You will get the same high level of service no matter which carrier you approach. Nowhere is it claimed that inflight products are identical, they are of the same "high standard" but not materially the same.