In the commercial world, airlines use a Configuration Deviation List (CDL) and Minimum Equipment List (MEL) to ensure the legality of a flight with missing/defective items.
The general philosophy is that the aircraft can take off with defects provided the systems are never degraded below that required by the CAA for
type certification.
A CDL/MEL never mentions items that are always required (eg wings, engines, landing gear etc) and items that don't affect airworthiness like carpets or pen holders.
Other items are then catalogued on whether they may be needed for a particular flight, so if your NAV radio is broken and you are flying VFR in VMC then there would be no need for the radio to work.
There is some useful information in
CAP747 Mandatory Requirements for Airworthiness.