Very shortly the EU Law will say something like this:
NCO.IDE.A.105 Minimum equipment for flight
A flight shall not be commenced when any of the aeroplane instruments, items of equipment or functions required for the intended flight are inoperative or missing, unless:
(a) the aeroplane is operated in accordance with the MEL, if established; or
(b) the aeroplane is subject to a permit to fly issued in accordance with the applicable airworthiness requirements.