Terrorism is a word with a specific meaning. Like most -isms, it means a political/social agenda or belief system. Other examples are socialism, conservatism, Islamism (the belief that a society should be run on Islamic lines), etc.
Terrorism does not mean "terrifying violence". Some people might use it that way, but that doesn't make it so. For a crime to be terrorism, there has to be political or ideological intent behind it. Owning chemicals or literature that are also used by terrorist organisations doesn't make you a terrorist, any more than a drug dealer who owns a submachine gun is anything more than a violent drug dealer.
Rudakubana was referred to Prevent, which is an explicitly anti-terrorism programme, with connections to the intelligence services. It's right there on the government website: "The aim of Prevent is to stop people from becoming terrorists or supporting terrorism." If they didn't take action it will be because there was no evidence that he was a potential terrorist, because he was not pursuing any agenda. He was just a violent individual. We could widen the remit of Prevent to include "anyone who might commit extreme violence one day", but that would make their job 100 times bigger, because there are very few markers for that.
We might like him to have had an agenda because that would have perhaps helped us make sense of it. It is frustrating to read that something so horrible "doesn't count" as terrorism, because either way three little girls are dead. But those are the political choices we make as a society. We could decide to apply the provisions of anti-terrorism laws to every murder, or every assault, or every robbery, and we would momentarily feel like we were "doing something about criminals", but we might not like the consequences for the way we live.