The reason? For almost 100 years now, England has had the most powerful right-to-silence of any country in the world, and dating before that there are hundreds of years of well established common law on the subject.
In England and Wales, you don't have to answer any question to anyone. The one proven exception is under tax investigation, however any evidence of criminal acts disclosed is inadmissible in court.
The 2000 R.I.P. Act supposedly made it illegal to withhold a requested key to protected (encrypted) information. However, to my knowledge this is untested in court. Although AFAIK the law is still too broad to be untestable in court, because under common law you can pick and choose when to talk (unlike Canada where if you talk, you have to talk), so I believe it's still unproven that you have to disclose it if you've never had a dialogue with the police, nor has it been tested against the privileged against self-incrimination. So potentially any evidence gained under this law may be inadmissible in court if it incriminates you.