I think you will find that the beacon is offically an 'anti-collision' beacon specified in the (UK) Air Naviagtion Order as a light that has to be displayed in flight (Flight being defined as the time the aircraft first moves under its own power with the intention of flight etc etc)
It is however now also used as an indication that the aircraft is live and is about to start although legally, as far as I remember, does not need to be displayed until the aircraft first moves.
As for switching it on, it will be an item on the pre-start checklist. Once the ground crew have confirmed that the aircraft is secure and ATC start clearance given, the checklist will be completed, the light will be on and the engines will start.
Update! - having just checked the ANO red or white anti-collision light also has to be displayed on apron whislt engines running.
Last edited by This is a crisis; 17th November 2009 at 08:39.