15 minor injuries? In general, that means cuts and scratches, or a minor undisplaced fracture (wrist etc.. painful but not anything that cant be fixed in short order in any A&E) From a professional point of view, I'm at the other end of alerts... Majax for hospitals. It will be interesting to see the extent of the injuries.
I'm sure that the cabin crew know perfectly well the inevitability of injury by getting people out of the aircraft by the slides. They do not do it lightly. I do not think for one moment that the flight crew gave the instruction for such an evacuation lightly either.
Sounds like a completely professional handling of the situation. The London hospitals did get the alert as planned, and acted with commendable speed. I'm sorry if some people had their surgery cancelled for today.