From the UK Government website:
The police arrest procedure
If you’re arrested the police must:
- identify themselves as the police
- tell you that you’re being arrested
- tell you what crime they think you’ve committed
- explain why it’s necessary to arrest you
- explain to you that you’re not free to leave
From the current news of the protests over the coronation:
Anti-monarchy group Republic's chief arrested at Coronation protest
Six demonstrators, including Mr Smith, were arrested by police while unloading placards near the Coronation procession route, the group said.
Republic posted photos of officers taking details from them on Twitter.
"So much for the right to peaceful protest," the group said, adding the officers would not give the reasons for their arrest and confirmed their CEO was among them.
Given that the police identify themselves as the police, tell you that you’re being arrested and explain to you that you’re not free to leave but do not tell you what crime they think you’ve committed or explain why it’s necessary to arrest you even after that information is requested and enough time has past that any other urgent business has been conducted, is that a legal arrest or not?
A similar case where it was ruled the the arrest was not legal would be Police inspector Dean Gittoes where a police officer was found guilty of assault for making an unlawful arrest. This would indicate the officers involved could face legal consequences if the arrest was not legal.