1. Obstruction of a Police Officer
The threat to arrest for obstruction is widely used by the police at demonstrations.
Under the Police Act 1996 s89 it is an offence to resist or wilfully obstruct a constable in the execution of his/her duty.
Willful obstruction of a police officer means doing any act which makes it more difficult for the officer to carry out their lawful duty e.g. stopping them doing something, de-arresting someone, deliberately misleading them, or giving a false name and/or address.
Being limp makes it more difficult for a police officer to move you, and is not obstruction.
The maximum sentence for obstruction of a PC is a Level 3 fine, currently £1000. First time offenders would likely receive a much lower fine than this. In all cases, fines are partly based on your weekly disposable income.