From CAP 694:
Flight Plan (FPL)
Specified information provided to air traffic services units, relative to an intended flight or portion of a flight of an aircraft (ICAO Annex 2 / ICAO Doc 4444).
No mention here of necessarily having to fill in a form though, as Backpacker and OhNoCb have said, there may be regional variations as to how a flight plan should be filed.