I don't believe IFR needs to be explicitly stated on the C of A, but the C of A incorporates any limitations in the AFM.
In the UK:
Dual altimeters (according to Schedule 4) required for:
* All IFR in A/B/C airspace
* Public transport under IFR except if outside controlled airspace and MTWA less than 1150 kg
* Public transport at night except if MTWA less than 1150 kg
Dual VOR (aaccording to Schedule 5) required for:
* Public transport under IFR
* Public transport under VFR if multi-engine