It depends on what you mean by a Flight Plan; there are two categories:-
i) Full flight plans; the information required on Form CA48/RAF2919
ii) Abbreviated flight plans; the limited information required to obtain a clearance for a portion of a flight (e.g. flying in a control zone, crossing an airway) filed either on the RTF, or by telephone prior to take-off.
Pilots are required to file flight plans for all IFR flights within controlled airspace (Class A-E), and for all VFR flights within Class B, C and D airspace.