PACE made a big difference. Instead of verballing (inventing admissions) and getting suspects to sign false confessions, the police had to have interviews on tape, and lawyers' or their representatives present in interviews. This severely limited their previous practices.
And the number of times violence was used against suspects was extraordinary. There was one police station in North London which must have had the most dangerous set of steps in Europe, judging by the number of times suspects "fell down" them. And , uniquely, it had an extraordinary number of suspects who decided to rush headlong at the cell door "just at the very moment as I was closing it , sir". Ah, so that's how he got head injuries. I had one case where the police broke the arrested man's back on the way to the police station. He'd been seen, by passing officers, to be winding up a new constable by dropping litter (not an arrestable offence). Those officers invented an assault on the constable, arrested the man for that, and taught him a lesson in the van back to the station.
And until Robert Mark became Commissioner corruption was rife in some units . Even then, complaining was often pointless because officers stuck together, wouldn't speak against colleagues.
Nowadays, the police are much better and more tightly regulated.