Following the TSA’s lead to disallow passengers at certain airports from boarding U.S.-bound flights with uncharged devices, the U.K. Department for Transport is also now requiring some international flights flying in and out of the country to not permit devices to be handcarried onboard if they can’t be powered up.
In line with the US advice, passengers on some routes into and out of the UK may now also be required to show that electronic devices in their hand luggage are powered up or face not being allowed to bring the device onto the aircraft. Passengers flying into or out of the UK are therefore advised to make sure electronic devices being carried in their hand luggage are charged before they travel.
On a related note, a list of electronic devices and electrical items permitted in hand luggage in flights flying in and out of the U.K. can be found here.
Regarding which routes will be affected, the department declines to specify for ‘obvious reasons’.
Source: U.K. Department for Transport