https://www.gov.uk/hand-luggage-restric ... ical-items
Flying to the US
Make sure your electronic devices are charged before you travel. If your device doesn’t switch on, you won’t be allowed to bring it onto the aircraft.
BBC
http://www.bbc.co.uk/news/technology-28193654
The authorities are not providing a specific list of the devices now subject to power-on checks.
However, the US Transport Security Administration has singled out mobile phones as one type of device affected, and the UK's hand luggage rules also mention tablets, MP3 players, cameras and electric shavers as other examples.