Police officers are allowed to use force in certain situations — but never as a matter of routine, and never more than the situation genuinely requires. The standard is that any force used must be necessary, proportionate and reasonable in the circumstances.
"Necessary" means there was no realistic, less forceful option that would have worked. "Proportionate" means the level of force matches the seriousness of what the officer is dealing with. "Reasonable" means another person, looking at the facts the officer honestly believed at the time, would consider the response justified.
Officers are expected to be able to account for and justify any force they use, and to use the minimum needed to achieve a lawful aim — then stop. Force that continues after it is no longer necessary, or that is used to punish or intimidate, falls outside the standard.
If you believe an officer used excessive force, you can complain to the force's professional standards department, and serious matters can be referred to the Independent Office for Police Conduct (IOPC).
This is a plain-English summary. For the exact wording, read the official Code of Ethics.