Counter terrorism and counter espionage are essentially the same thing. An enemy has infiltrated their country, and it MI5 must find them and prevent them from executing operations against the British people.
Terrorists are non-uniformed combatants and under the Geneva Conventions are not entitled to protection. While terrorist acts technically crimes in civilian law, the scope of terrorist acts are such that they are acts of war, and the law must treat terrorists as agents of a foreign power. This includes charging those who hold British citizenship with treason.
All of this money spent fighting terrorism wnd the answer right in front of them. Stop Islamics from entering your country and be sure the ones already there are watched closely. When they make a treasonous move , Pounce on them and treat them harshly, by imprisonment and exportation.
It needs to be done in Europe and it needs to be done here.