Unfortunately they thought of that too. (This is from an article linked within):

The policy document reveals that the owner of the device must not be allowed to gain access to the device until an officer has completed the examination. That means mobile devices must be put into flight mode and have their SIM card removed, and laptops must have batteries removed if possible, rather than just shut down.

It also suggests that for Apple iPhones where the passcode is not known, officers consider holding any computer the owner has.