
As we reported previously, the TSA in the United States is now requiring all passengers traveling into the U.S. from international destinations to power on their smartphones and tablets before being allowed to board their flights. This is an effort to thwart potential terrorist activity by hiding a bomb in a smartphone.
By requiring passengers to turn on and show the device functioning it basically proves (for the most part) that it isn't a bomb. Of course, this means passengers need to have the forethought to charge their devices ahead of time. Effectively, this bans uncharged devices from flights. It now looks like airlines in the UK are following the TSA lead as they have begun instituting the policy as well.
Here's a quote with the details,
A Department for Transport spokesperson said:
The UK government keeps aviation security under constant review in conjunction with international partners and the aviation industry.
On 2 July we announced we were stepping up some of our aviation security measures. 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.
For obvious reason we will not be commenting in detail on the routes affected. We will work with the aviation industry to minimise disruption as far as possible.
The UK has some of the most robust aviation security measures in the world and we will continue to take all the steps necessary to ensure that public safety is maintained.
Consider yourselves officially informed.

Source: https://www.gov.uk/government/news/aviation-security-update