UK law runs on the "reasonable" adjective. The court would ask itself if it is "reasonable" in their own opinion that you don't have the info.

Typically it's applied to cases where the information is clearly available, like a drug dealer not remembering his daily driver laptop password, or refusing biometrics unlock. Not, we found this thumb drive within a mile of your house, decrypt it or else.

But of course the standard of "reasonableness" is murky, and you'll find plenty of cases that revolve on such contested judgment.