The current breakdown in terms of where posters appear to side on this, on the narrow but key question of whether BA were “right” to deny my wife boarding, is as follows:
Andriyko is not siding with BA.
Andriyko said that BA were right to conclude that your wife did not have proper documents for the destination on the ticket. The broader (and main) question here is whether BA and airlines in general are allowed by law to demand documents for the destination on the ticket only and to disregard any other travel arrangements the passenger may have. I don't have an answer to that question. Given the implications for airlines, I can see how a court can side with BA. I can also see how it can rule against BA. When the broader question is answered, it'll help many passengers. When you submit a claim under UK261, it'll be interesting to hear how BA respond.