The short of it: Attention Marks and Spencer's, Oxford Street: Stop being stupid with your unducted portable air conditioners that blow hot air out of the top. Either duct the hot air out of the building or turn them off and give it up. You are making the building hotter. The staff are sweating. The planet is being needlessly warmed.

The long of it: A perfect, 100% efficient refrigeration unit would produce one unit of cold air out the one end for every one unit of hot air that it produced from the other end. If you vented both into the room you'd be right back where you started.

However the second law of thermodynamics (Heat cannot of itself pass from one body to a hotter body) states that you not only cannot win, but that you cannot break even. An actual aircon will be less than 100% efficient, probably a lot less. It will produce more hot air than it does cool air. Vent both into the same room and you might as well run a heater. You seem to have tried to fix the situation by bringing in even more portable aircons to cope with the heat, but they're not going to help. They're making it worse. You're wasting electricity; you're losing business as people flee the shop; you're damaging our environment and you're being stupid.