If he's smart, and has a smart solicitor, he "should" be ok. I can see a decent defence for him in a mix of 'instant arming' and 'pre-emptive strike'.
Though you are right, this shit shouldn't happen and it does anger me. The poor guy should get a medal.
Reading the comments afterwards, it still isn't clear to the general public that it isn't the police that decide on charging, the decision making body is the CPS. Police are mearly the conduit to get them to court. Police don't really have any decision making priviledges anymore. Otherwise we could excersise some common sense, especially in cases like this.