Legal/Illegal has lost all usefulness as an arbiter of anything, centuries of lawmaking has made a joke of the rule of law, with the years since 9/11 being a regular punchline.  

The British Security Industry Authority (BSIA) recently estimated that there could be up to 5.9 million surveillance cameras in the country -- or one camera for every 11 Britons. Most were not installed by the government, but by companies and private citizens. One wonders who even has the time to look at these images.

While there is the occasional burst of resistance on the island, most just accept surveillance as the price of freedom.

[...]

The spies expect preemptive subservience and discretion from the country's press, and they often get what they want. There is no other explanation for the matter-of-factness with which government officials and GCHQ employees contacted Guardian Editor-in-Chief Alan Rusbridger to demand the surrender or destruction of hard drives. What is surprising is the self-assurance that led the powerful to believe that none of this would ever come to light. According to the newspaper, after the hard drives had been destroyed in the Guardian's basement, an intelligence agent joked: "We can call off the black helicopters."

1 Terrorism: interpretation.

(1)In this Act "terrorism" means the use or threat of action where--
(a)the action falls within subsection (2),
(b)the use or threat is designed to influence the government [F1or an international governmental organisation]F1 or to intimidate the public or a section of the public, and
(c)the use or threat is made for the purpose of advancing a political, religious [F2, racial]F2 or ideological cause.
(2)Action falls within this subsection if it--
(a)involves serious violence against a person,
(b)involves serious damage to property,
(c)endangers a person's life, other than that of the person committing the action,
(d)creates a serious risk to the health or safety of the public or a section of the public, or
(e)is designed seriously to interfere with or seriously to disrupt an electronic system.
(3)The use or threat of action falling within subsection (2) which involves the use of firearms or explosives is terrorism whether or not subsection (1)(b) is satisfied.
(4)In this section--
(a)"action" includes action outside the United Kingdom,
(b)a reference to any person or to property is a reference to any person, or to property, wherever situated,
(c)a reference to the public includes a reference to the public of a country other than the United Kingdom, and
(d)"the government" means the government of the United Kingdom, of a Part of the United Kingdom or of a country other than the United Kingdom.
(5)In this Act a reference to action taken for the purposes of terrorism includes a reference to action taken for the benefit of a proscribed organisation.

Paragraph 2(4) goes on to provide that an examining officer may exercise the power  whether or not he or she has grounds for suspecting that a person falls within section 40(1)(b).  This means that there does not actually need to be a reasonable suspicion.  However, paragraph 2(4) does not negate the requirement that the power be exercised for the purpose specified.