Section 31 of the Police and Criminal Evidence Act:

  Where—

    (a) a person—

      (i) has been arrested for an offence; and

      (ii) is at a police station in consequence of that arrest; and

    (b) it appears to a constable that, if he were released from that arrest, he would be liable to arrest for some other offence,

  he shall be arrested for that other offence.

That's because arresting someone isn't so much a matter of their physical custody as initiation of a legal process involving an assertion of custody. Think of it like process serving in civil cases, where it's easy to imagine multiple lawsuits proceeding in parallel.

It's also used as an unofficial form of punishment.

Or "official" form... see Japan's legal system, where prosecutors interrogate those arrested without legal council, and the right to post bail can be an award for a confession.

see: https://www.cnn.com/2019/04/03/business/carlos-ghosn-nissan-...