[rahimnathwani]

From that page: “You do not have to say anything. But, it may harm your defence if you do not mention when questioned something which you later rely on in court. Anything you do say may be given in evidence.”

IANAL, but my understanding is that if the question is "What's your Facebook password?" and you refuse to give it, then the prosecution can mention this fact in a trial. But this by itself is not a problem for you. However, if you claim in court that there's evidence in your Facebook feed that shows you did not commit the crime (e.g. photos of you elsewhere at the time), then the jury might hold it against you that you didn't allow the police to access your Facebook account at the time, and they might then question the strength of this evidence.

Better explanation here: https://www.quora.com/UK-police-constables-say-it-may-harm-y...