[Truth29]

"However, it wouldn’t have been necessary to arrest them, at this point. If the police had gotten the young men’s vitals at that point, the police could have let them go and picked them up later if it was warranted."

The police collected their fingerprints and DNA. The Brits have been on a tear for a universal database and maybe they used this as a pretext for mandatory collection.

[Jonty30]

That’s certainly possible. I know Britainisn’t as freedom minded as Canada or the US.