The "different tickets" part seems to have an unstated disclaimer "as long as it isn't on a competitor". If same PNR, no problem interlining (obviously), but if separate PNRs BA seems to view it as a favour and not a part of the contract of carriage, thus the hit and miss experience with interlining. BA has adopted several such strange tendencies in the past year. Really deplorable as these are minor matters which don't involve much revenue or cost and create a smudge on the airline, besmirching its many admirable qualities. In aggregate it really does a lot of damage to BA, particularly when they publish different rules on the website than they actually implement, and dismiss pax who provide printouts of the rules from ba.com as "these aren't our procedures, sir, we don't publish them on the web". I guess ba.com isn't really part of the airline, at least in the minds of some employees.