Mkuu Prof good to see you bro,
The definition of An act of aggression is Janus-faced. What Theresa said can amount to an act of aggression, because it doesn't only need A legitimate use of armed forces of one state making military incursion into another states territory.
An act of aggression can be done even by state agents or even armed groups so long as they work under the umbrella of that country sending them. This was very clear in the case of Nicaragua v United States..The (International Court of Justice) ICJ was of the opinion that America was an aggressor even thou she did not send her armed forces but instead she armed the Sandinista rebels (So the jurisprudence of the Court with regard to Military Aggression was very broadened)
The International Criminal Tribunal for the Former Yugoslavia (ICTY) set a firm boundaries in the case of Dusko Tadič v The Prosecutor by giving out the tests of measuring whether the an armed group acts on behalf of another state. There is what we call Effective Control and Overall Control Tests (I advice you to read the case, its very Interesting. I am willing to provide you with materials for this if you may be interested )
NB: Henceforth, do not forget that an Act of aggression invokes double penalty under International law. At first a Nation State gets punished for international wrongful act under the Public International Law Principle of STATE RESPONSIBILITY. A State may be sanctioned or isolated by the international community or sometimes the security council may invoke Chapter Seven VII of the United Nations Charter 1945 and authorize military action against it. Refer the issue of North Korea in 1950, after she invaded South Korea by the crossing the 38th Parallel. On the other hand refer the Lockerbie Incident of 1988 where Libyan remote state operatives bombed the Pan-Am flight 103 (Cargo Plane) but the whole State suffered through sanctions and Isolation. And finally you can read the Rainbow Warrior Case, where French spies sunk a Environmental activists ship which was there to stop French Nuclear Bombing: France as a state paid indemnity fee for reparations to the families who lost their beloved ones and the spies who caused the homicide where detained in a remote island for three years (but they got out earlier than this)
At second instance,
A Crime of aggression can target the state officials who have perpetrated or orchestrated the Crime of Aggression. And this is because Criminal Responsibility is Individual Responsibility (Here you can refer the Leipzig trials after The First World War and Nuremberg trials after the Second World War). But unfortunately, since 1945 Nuremberg and Far East Military Trials no other state official has ever been prosecuted for the crime of aggression.
The Rome Statute establishing the International Criminal Court (ICC) lists it down as one of the International Crimes but it is not effective and not is not used. It was supposed to have been passed it after the Ugandan Convention in 2015 (If i remember correctly) but state officials members to the treaty are very unwilling to pass it (It is very dangerous to allow the crime to be tried by the Court)
In the current case,
Russia as a state can be held liable for An act of aggression which invokes the Principle of State Responsibility specifically under the periphery of Imputability theory because the act was state sponsored (if the British can only prove the claim)
And if they want to prosecute Vladimir Putin and cronies. They can only accuse them of Crimes Against humanity but again its too difficult because Russia is not a signatory of The Rome Statute which creates the ICC. And the crime of aggression is not operational within the Statute or anywhere..[emoji23][emoji23][emoji23][emoji23][emoji23][emoji23]
NB: It just gets better and better......
And they only use state responsibility because that's the only argument they have (Yesterday Security Council meeting with regard to Salisbury Attacks was so hooooot, please watch it bro)