The flight was over four hours delayed. There was availability on the (also delayed, but not as badly) Stansted flight, so I would have expected the change to be at no charge. There should have been no commercial aspect to it. It just needed someone with the authority to make a brief phone-call and hand-write a new boarding card, resulting in a slightly happier customer. Not exactly rocket-science, hmm?
Really, lower cost should not be an excuse for failure to apply commonsense. Sadly, it appears to be.