In this answer Matthew mentions that Section 366 of the Communications Act 2003 grants the person executing the warrant the power to examine your equipment.
Specifically it says
give the person carrying out the examination or test all such assistance as that person may reasonably require for carrying it out.
and
A person is guilty of an offence if he—
(a)intentionally obstructs a person in the exercise of any power conferred on that person by virtue of a warrant under this section; or
(b)without reasonable excuse, fails to give any assistance that he is under a duty to give by virtue of subsection (7).
So say you have a locked mobile phone on you that requires a passcode to unlock. Such devices can receive broadcasts that require a TV licence, e.g. via iPlayer. Do you have to unlock it?
Would, for example, the argument that your phone or computer contains sensitive or personal data be a "reasonable excuse"? Such data could include embarrassing photos, other people's personal data and the like. In particular I can imagine they may want to view your browser history to see if you have visited iplayer.com.