Can they force you to unlock a phone? I recall that American judiciary do throw you in a cell until you do (forever, if you refused), but what about the Brits?
There are two different situations that have gone before the courts... One where they KNOW there is incriminating evidence on the hard drive, and one where they SUSPECT there is incriminating evidence. Both have had their day in court and there are several different outcomes.
There was one where Border Patrol found kiddy porn during an inspection on a laptop that was powered on and decrypted, but it locked when powered off, and the suspect refused to give the password, citing 5th amendment privilege. He was jailed on contempt, the judge not buying the "testifying agains one's self" 5th amendment argument because producing the password would not give the government something they didn't already know (that there was kiddy porn on the computer).
Several others have related to financial crimes where the government did NOT already know the contents of the drives - which were seized during warrant raids in an already encrypted state. These are the cases where providing a password IS tantamount to testifying against one's self and aiding in their own prosecution. There is a difference between being compelled to produce something physical (like a key to a locked safe) and being compelled to produce something that only exists in someone's mind (for example the cipher to paper diaries written in one's own personal code that are inside said safe). Of course there is nothing stopping the government from attempting to break encryption through their own brute force, but so far popular open-source encryption like TrueCrypt and VeraCrypt are proving impossible for the FBI to crack - thus they need an easy shortcut in the form of attempting to compel the suspect to give up the password. As I said, the appellate courts have actually gone both ways on this. A couple have upheld the contempt charges, while others have tossed them on 5th amendment grounds. None as of yet have made it to the Supreme Court. Meanwhile the government is trying to enforce "Key Escrow", which is basically a back door into any encrypted system (the San Bernardino iphone is their big poster child for that), pulling the rug from under those who would rather go to jail than help the government prosecute them.
