CAP 694 The Flight Planning Guide says
For IFR flights, this is the total estimated time required from take-off until arriving over the designated point from which it is intended that an Instrument Approach Procedure, defined by reference to navigation aids, will be commenced, or, if no navigation aid is associated with the destination aerodrome, until arriving over the destination aerodrome itself. For VFR flights, it will be the estimated total time required from take-off until arriving over the destination aerodrome.