Searches under the section you bolded are ONLY authorised when they think it will stop a terrorist act, not, because they just want to stop someone.
They SHOULD have the power to stop anyone if they think they are being suspicious or involved in terrorism.
However cops shouldn't and can't, as in its illegal to just use it as a blanket reason to stop and search anyone they choose. As in that video, the part I could bear to watch he said he wanted to search him under act 44, when asked what gives him the reasoning under act 44, he went "well, ummm, I dunno, I just want to search you".
Thats just not acceptable, literally, by the letter of the law that cop HAS TO have sufficient grounds to think someone is a terrorist, he clearly did not think he was a terrorist but simply wanted to search him so said he thought he was.