The TSA (Transport Security Administration) has implemented enhanced security measures at certain airports overseas for direct flights to the United States, including London Heathrow:

As the travelling public knows, all electronic devices are screened by security officers. During the security examination, officers may also ask that owners power up some devices, including cell phones. Powerless devices will not be permitted onboard the aircraft. THe traveller may also undergo additional screening.

TSA will continue to adjust security measures to ensure that travellers are guaranteed the highest levels of aviation security conducted as conveniently as possible.

You can find BA's advise here, including a FAQ, for travelling from the UK.

If you have any concerns regarding your flight, please contact you airline.