In UK air space you need ADF and DME if you are conducting a flight under IFR irrespective of whether or not the aircraft is on the G-register and irrespective of the other equipment you have installed.
And the source of this info is??
You can of course fly IFR in a non radio aircraft with no navaids at all - in VMC you can even do it safely (you do not have to be in IMC or CAS to fly IFR)
