The CAA has conducted research which provided evidence that a mobile phone transmission on-board an aircraft may interfere with equipment including communications, navigation and flight control systems. There is circumstantial evidence that portable electronic devices such as CD players and computer games can also cause interference. As a result there is a requirement that:
The use of mobile phones on board is prohibited once all of the aircraft doors are closed.
In addition, the use of personal electronic equipment by passengers, such as laptop computers, electronic games, calculators, CD players, cassette players, radios, TVs, video cameras and remote controlled toys, is prohibited during take-off and landing phases, when the passenger seat belt sign is on, and whenever the aircraft commander suspects that their use may be the cause of interference.
These are the CAA regs, all UK registered aircraft have to adhere to this policy at a minimum. If they choose to introduce stricter rregs for their airline, it still has to be approved by the CAA and documented in form of company orders.