I know that I am taking my life in my hands by arguing with a lawyer, but I believe that the noble Lord is referring to the Human Rights Act, which requires necessity and proportionality before the officer exercises the power of arrest. However, under the Police and Criminal Evidence Act, the constable can arrest somebody if they have reasonable cause to suspect that they may be about to commit an offence—which is what I have just said.
The advantage of legislating this way round, as proposed in the amendments, is that, if people are visiting sick or dying relatives, or are aid workers or journalists and have a genuine reason for travelling, they will not be committing an offence and will not be unreasonably deterred by the fear that they may be arrested, either on their way to or their return from a designated area.