While I am not normally one to espouse vigilantism, if anyone were trying this with one of my (grand)kids I'd bring some friends and finish it.
I agree. If somebody hurts my kid or grandkid they had better watch it. I believe that knife sales, even kitchen knives, are regulated in the UK. Guns are effective weapons and access to them would be a good thing for the law abiding folks in the UK.
I strongly approve of allowing the rule of law to prevail. It's only when the courts fail that I prefer to see justice attained by other means. Regardless of the path though, those who prey on children should receive swift justice - even if they also "pray" five times a day.