You sure seem to be right about the broader definition! But legal or not, it still seems absolutely crazy to classify this type of property damage as terrorism to me… I have a hard time to see how to justify that beyond, of course, the technicalities of the definition in the UK

I’ve read that it used to be “heels over head” as in upside-down, but then somehow the words got switched around (I found this page that claims the same thing: https://www.phrases.org.uk/meanings/head-over-heels.html ).