As far as I understand, this law was created to get very important information from really serious criminals. But if a person has materials that may get him into more serious troubles than 5 years jail-time it is in their interest not to provide encryption keys.
Perhaps 5 years is the time the UK security services believe is required to crack currently available encryption systems if necessary. So, you plan your truck bombing, get locked up for 5 years for not revealing the decryption keys, then get locked up for "life" for the planned terror acts.
Or do I miss something?