The simple reality is that the pre-PACE system only worked because the police bent the rules to amke sure there was sufficient evidence to convict people ("verbals", planting bits of evidence, etc.). EVERYONE involved with the criminal justice system knew it and connived in it as they knew the system only worked because of it. Defence lawyers and judges went through the motions of challenging the bent bits but all a bit half-heartedly. The police went through ritual denials and everyone moved on. Occasionally something went wrong and (usually) some copper was hung out to dry in a particular case and it was portrayed as something of a one-off ... and then everything went back to normal.
All this changed with PACE. Suddenly there were limits on detention times, lawyers in interviews, etc. Tape recorded interviews came in at the same time, so the crafted confessions / part-confessions in contemporaneous notes went. Lay visitors were introduced and suddenly cells could be randomly visited at any time by a citizens panel. In the space of a couple of years everything changed. Convictions plummeted and the incompetence of the system became plain for everyone to see.
Since then there have been loads of changes made to try and redress the balance. The police have changed their investigative methods to comply with the law and do their best within the rules (investigative interview (PEACE) training, for instance) but even that alone was not enough and changes to the law have followed (e.g. allowing inferences to be drawn from silence) and it's probably just about workable again now ... but it's taken over twenty years!