The House of Commons Library has produced a research briefing for MPs and their staff to help them to deal with casework involving dissatisfaction with the police including:

• understanding the police complaints system
• where to find further information about police complaints
• how to help constituents make complaints
• how to help constituents who have already made complaints
• and much more

You can find it here: https://commonslibrary.parliament.uk/research-briefings/cbp-9053/