Like Gringo I too made hydrogen, acetylene, gunpowder, and flash powder growing up. My dad was part and parcel of it. He provided the recipes and material and made sure we built things safely. I guess by todays standards he would be considered a criminal and bad parent.
We assembled the materials into small devices using balloons, model airplanes, paint cans, tin cans, empty CO2 cartridges and model rocket engines. Model rocket fuses were the preferred ignition mechanism because they could be detonated on command from a distance.
We used them to make noise, blow holes in the ground, destroy model airplanes and even to try to remove small stumps. A tractor is a better at removing stumps, but not quite as much fun. Yes we used them to destroy property, our property, so who the heck cares.
I tend to agree with Martlet and a few others that this was an overreaction, but I'm not surprised. As far as possessing such devices I think one should no more have to justify their possession than they would a gun, or a civil war cannonball. It is not the government's job to keep us safe from ourselves.
I would also suggest that in a strictly logical world the illegality of the devices might be questionable. The applicable MGl seems to indicate intent to harm or unusual explosiveness. I've not heard any intent to harm in the reports. If any threat was made then throw the book at him. If not, then not. AFAIK there is no law against stupidity in not keeping your mouth shut.
And who is to say what level of explosiveness is "unusual". A nuke, yes, but some might consider the amount of explosiveness in a single loaded round to be unusual. For those that claim that reloading or black powder is different I would just note that we sometimes used, and preferred, store-bought black powder; guess what it was sold for.
As far as the person who reports,
I'm a bit biased, but in this case I think it a bit strong to call him a rat, at least not without more info. I don't think I would have not have called the police unless there was something else besides the mere existence of the devices to make me think he might be a danger to others.