The legal issue is the border power used, not Yaxley-Lennon's own description of the episode. The Guardian reported that police said they stopped him and seized his phones after he disembarked at Heathrow. The Irish Times reported that the Metropolitan Police said officers stopped a man in his 40s at about 5pm on June 13 after he returned to the UK from Russia via Turkey.
Schedule 3 is a hostile-activity port and border examination power, meaning questioning at the border to test whether a person appears to have been involved in activity linked to a foreign power or other hostile actor. The legislation says an examining officer may question a person for that purpose. The power is separate from a criminal charge and does not itself mean the person has been charged with an offence.
The revised Schedule 3 code of practice says an examining officer must tell the person that they are being examined under Schedule 3. It also says the officer has the power to detain the person for the examination and should explain relevant information in a way the person can understand where practicable.
That distinction matters in this case. Sky News reported that Yaxley-Lennon said officers held him for the best part of three hours and seized his phones; the Guardian described his account as about three hours. The checked sources did not show that police charged him, arrested him under a separate offence, or accused him in court of hostile activity arising from the Heathrow stop.
The legislation and code set time limits. Paragraph 5 of Schedule 3 says an examining officer may question a person for up to one hour from the time the person is first questioned unless the officer detains that person under paragraph 4. A person detained under paragraph 4 must be released by six hours from the time they were first questioned. The code says questioning that requires a criminal caution should take place outside the Schedule 3 examination process.