Legal definition of Terrorism definition via MI5 and CPS. It's pretty broad tbh.
In this is Act "terrorism" means the use or threat of action where-
The use or threat is designed to influence the government or an international governmental organisation, or to intimidate the public or a section of the public, and
the use or threat is made for the purpose of advancing a political, religious, racial, or ideological cause.
Action falls within this subsection if it
involves serious violence against a person,
Involves serious damage to property,
Endangers a person's life, other than that of the person committing the action,
Creates a serious risk to the health or safety of the public or a section of the public.