Notice that I said, "Miranda was detained under schedule 7 of the Terrorism Act", this statement does not contradict your statement.
I am not a lawyer and I am in no way familiar with UK law, but my reading of the law is given below:
tdlr; Section 7 applies people that the officer wishes to determine are terrorists defined in Section 40(1)(b)[1]. It does not require evidence, but the purpose is to determine if the person is, in fact, a terrorist.
[1]: http://www.legislation.gov.uk/ukpga/2000/11/section/40
Section 7 states: "For the purpose of satisfying himself whether there are any persons whom he may wish to question under paragraph 2 an examining officer may—"
Paragraph 2 states: "An examining officer may question a person to whom this paragraph applies for the purpose of determining whether he appears to be a person falling within section 40(1)(b)."
Section 40(1) (b) states: "In this Part “terrorist” means a person who— (a)has committed an offence under any of sections 11, 12, 15 to 18, 54 and 56 to 63, or (b)is or has been concerned in the commission, preparation or instigation of acts of terrorism."