Earlier quoted context omitted.
The UK stopped being a democracy a long time ago. In the UK if you keep your mouth shut during an investigation, it may harm your defense. There is zero right against self-incrimination. It is also the place where you can be jailed for not providing the PIN to your phone. I could go on.
You WILL be charged for not giving the PIN to your phone in an investigation. It’s a strict liability offence which means there’s no legal defence. The only realistic protection here is a “decoy” partition, a partition that provides access to a separate installation of the OS. That gives the device owner plausible deniability that their real system even exists.
Again, no, not just "in an investigation".
It has to be an investigation that has reached the point where they have gone to a judge and got permission to serve written notice. The judge will ask them if there is a necessary and proportionate reason. There is then a timeframe in which you have to respond.
Police can't simply demand your PIN and charge you if you don't comply.