At this early stage, the incident is not being treated as terrorism

The UK legal definition of terrorism:

The Terrorism Act 2006 uses the definition of terrorism contained in the Terrorism Act 2000. Section 34 amends that definition slightly, to include specific types of actions against international governmental organisations, such as the UN. The definition in the Terrorism Act 2000 (as amended) states:

(1) In this Act “terrorism” means the use or threat of action where:
the action falls within subsection (2)
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
the use or threat is made for the purpose of advancing a political, religious or ideological cause.

(2) 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
is designed seriously to interfere with or seriously to disrupt an electronic system

I cannot see how this is not domestic terrorism.