Would you?

In the UK, they can't force you to speak, but not answering questions can be interpreted as evasion / "adverse interference".

They do not need a warrant to search your phone. Not providing them with a PIN can be a separate charge, with up to 2 years imprisonment.

They can hold you up to 36 hrs if the police decide so. Up to 14 days without charge under modern broad "anti terrorism" laws.