The problem seems to be endemic of the low service carriers, it was pretty much the "baggage dance":

"You want to charge me £56 for hold luggage, but I can get it on for free, and it's more likely to be free if its hold luggage size"

That however, is based on the idea of enforcement. BA has half arsed this one by charging for hold luggage without enforcing the hand luggage rules. Passengers are playing the game. BA hasn't thought about it...