It's a theoretical thing under the Regulation of Investigatory Powers Act, IIR. It hasn't been tested. In practice under the law it'd probably be a stretch under a sensible judiciary since you can't prove a negative and thus can't prove you don't know something.

In a number of countries there is a defined offense, like in Australia if they don't believe you they can jail you for six months under the Cybercrime Act, 2001, or possibly 2 years (failure to obey a court order under the Crimes Act, 1914).

A story from the US:

https://nakedsecurity.sophos.com/2016/04/28/suspect-who-wont...