@AssassinatedBeauty
The UK Terrorism Act from 2000 defines terrorism as the use or threat made for the purpose of advancing a political, religious, racial or ideological cause (with "racial" having been added in 2009). The incel movement is an ideological cause. It hasn't been just about politics for the last 20 odd years or so.
I agree that the US (and others) has a motive for wanting to rename its enemies as terrorists to avoid having to treat them as prisoners of war, and to enable practices like extraordinary rendition. But that is nothing to do with this.
I disagree that the general widening of the term is unrelated by the move to allow governments to treat a wider and wider group of activities as terrorism. The government of the UK hasn't been innocent in this either.
But even within this definition, you have to differentiate things like criminal acts from terrorism, and the marker is around trying to create political, or you might say institutional, change. So politics, even in the larger sense that includes religion or ideology.
But that's not the same as someone who is say, trying to kill women because he hates them, or even a member of a religious or political group which is just trying to strike a direct blow for their cause.
If terrorism isn't differentiated from warfare, or insurgency, or criminal acts, you lose the ability to talk about any of those things clearly, and to deal with them under the law.
A young fellow, even one frequenting incel websites, who goes out and commits a crime in his own country, is a criminal, and should be subject to the criminal justice system, including it's protections, As soon as he's labeled as a terrorist, there is room to treat him differently, including removing those protections. There are a lot of implications there.
It's also not just a UK issue, as terms like terrorism have international implications.