The law says that the police can only arrest you if:
An arrest for any other reason is unlawful. For example, if the police arrest you just to scare you, this is unlawful. An arrest is also unlawful if the police do not obey the rules about arrest.
If the police arrest you, you can usually pay bail money to the police or the court, and go home until the date of your court case. (See page129 Bail)