The police need reasonable grounds to suspect you’re involved in a crime for which your arrest is necessary. The police must identify themselves as the police, tell you you're being arrested and explain why, tell you what crime they think you've committed, and explain you are not free to leave.
You don’t have to answer the questions. The police must explain this to you by reading you the police caution: “You do not have to say anything. But, it may harm your defence if you do not mention when questioned something which you later rely on in court. Anything you do say may be given in evidence.”
The police can hold you for up to 24 hours before they have to charge you with a crime or release you. They can apply to hold you for up to 36 or 96 hours if you’re suspected of a serious crime, eg murder. You can be held without charge for up to 14 days If you’re arrested under the Terrorism Act.
You have the right to free legal advice. The right to tell someone where you are. The right to have medical help if you're ill, the right to see the police rules, and The right to food, water and use of a toilet. Vulnerable adults and minors have the right for an appropriate adult to be present at all times.