At protests and demonstrations, the police often threaten to arrest people for the offence of 'Obstructing a Police Officer in the Execution of their Duty'. The offence of assaulting, resisting or wilfully obstructing a police officer in the execution of their duty is an offence under the Police Act 1996.
These are summary offences, meaning that you can only be tried in a Magistrates Court and therefore can not have a jury trial.
This act applies to police officers acting in the execution of his duty, or people assisting a constable in the execution of their duty. There are also separate identical offences laid out in Section 46 of the Police Reform Act 2002 that apply to Police Community Support Officers.
It is also made clear in the CPS guidance that the offence of assaulting a police officer should be charged under Section 1 of the Assaults on Emergency Workers (Offences) Act 2018, which is an either way offence (meaning it can be heard in both the Magistrates or Crown Court, where you can have a jury trial).
The prosecution must prove that a person assaulted or resisted or wilfully obstructed a police officer who was acting in the execution of their lawful duty.
Assaulting a police officer is where one intentionally or recklessly causes a police officer to apprehend or sustain immediate and unlawful violence. There does not necessarily have to be any actual injury done to the officer.
The offence of wilfully obstructing a police officer is where one does something that makes it more difficult to carry out their lawful duty. For example, if one were to stop them from doing something that is part of their lawful duty, such as arresting someone, or deliberately misleading them or by giving them a false name and/or address. It is worth noting that simply not giving your details does not amount to an obstruction.
By going limp when an officer is trying to arrest you, it will make it more difficult for the police officer(s) to move you, but this should not amount to resisting an officer.
Under the Police Act 1996, the maximum sentence for assaulting a police officer in the execution of their duty is 6 months' imprisonment or an unlimited fine or both. However under the Assaults on Emergency Workers (Offences) Act 2018, the maximum sentence for assaulting an emergency worker is 1 year imprisonment or a fine or both.
The maximum sentence for resisting or obstructing a police officer in the execution of their duty is 1 months' imprisonment or a fine of £1000 or both.
Sentencing starting point: Community Punishment Order