Could of come under endangering the life of other travellers if you wanna look at it by stopping it they have technically blocked off a exit from the station.
No. There will always be altenative exits from the stations, afterall escalators fail from time to time. If it was the case then London Underground would be endangering passengers every time an escalator failed.
Actually that is one of the problems in a case like this. If the idiot who pressed the stop is liable for the injury, then London Underground could be liable for an injury if an escalator stopped due to a fault. If it was a powercut, the electricity company could be liable, or perhaps London Underground for not maintaining backup power.
From a criminal point of view it seems the injury wasn't taken into account. The byelaw offense would be incorrect use of the stop button. The public order offense is likely as he knew (or should have known) that pressing the button would cause disruption. Whether the latter will stand in court though is another matter.
From a civil liability point of view, I would expect some liability would be placed on the injured party, afterall I find it hard to believe if they were following the correct proceedure, i.e standing on the right and holding the handrail, then an injury would have occured, infact were they running down the right hand side as many do it could be argued that they may have been injured regardless.