Immigration Enforcement have power of arrest for this circumstance. The Information Commissioner has uniformed officers with limited police-type powers for their area of work too.
And I'd prefer no speculative high-drama arrests of people who are following the rules and not breaking the law.
> María, 25, from Valencia, said that like many of those detained, she thought she was free to explore the job market at least until October
This sounds like coming to the UK for work without a visa, in which case it is correct in law to be detained.
I think the practice of detention for up to 7 days before being deported is a way to inflict a punishment to deter people trying. Indeed, if the only thing you risk if caught is being turned away at the airport then people would just try and try, why not?
There are articles from sources a bit more balanced than The Guardian: https://www.politico.eu/article/eu-citizens-detained-uk-work...