If You Are Arrested
Stay calm. Say "No comment" to every question. This is not suspicious — it is your legal right under Article 6 ECHR. You will be offered a free duty solicitor at the station; always accept. Do not sign anything without legal advice.
Complaints That Actually Work
Internal complaints to the force that wronged you are largely pointless. Go directly to the Independent Office for Police Conduct (IOPC). If you have video evidence, attach it. Lodge your complaint within 12 months, though sooner is better.
Useful links:
Know Your PACE Codes
The Police and Criminal Evidence Act (PACE) 1984 has Codes of Practice (A–H). Code A covers stop and search. Code C covers detention. Code G covers arrest. If an officer breaches these codes, their evidence may be excluded from court under Section 78 PACE.