The section 24A conditions
Prevent physical injury, prevent property loss or damage, prevent absconding before a constable arrives, or prevent them causing loss or damage to themselves. Absent all four, the arrest is unlawful.
What 'reasonably practicable' means
If the police are en route in five minutes and the suspect is contained peacefully, a formal arrest by you is rarely required. Detention pending police arrival, done proportionately, is a different question.
Doing it lawfully
State clearly that they are under citizen's arrest and why. Use minimum necessary force. Call 999. Preserve evidence and witnesses. Do not question them beyond identity.