I wouldn't use one data sample to say that BA don't want customers!

Maybe all the cheap flights were already full that day on BA. Maybe KL was having a sale? Maybe KL are loosing money on that fare? Maybe the KL "complete package" (baggage/catering/connecting times etc) isn't as good as BA? Maybe (probably) the BA LHR-ABZ flights are full of high paying business people connecting in Club to longhual flights, so it's just not in BAs interest to sell cheap connections to shorthual destinations.

At the end of the day, BA and KL are both free to charge what they want, and you as a customer can choose who to buy from. If BA is charging too much for the market, then i'm sure they'll readjust their pricing. If you want to fly BA, then you have to pay what BA ask.