I have flown NDB procedures in the UK on an N-reg having such an IFR approved mounted panel GPS without an ADF onboard. Is that in violation of a regulation ?
Until the very recent change by the UK CAA (to which I can't find the link right now),
yes.
But I don't think the UK is the only country requiring the carriage of an ADF for IFR in CAS. Last time I looked (a few years ago) Switzerland was the same, and Germany mandated it for something like night VFR.
Equipment carriage regs are an
airspace requirement so they catch all aircraft flying there, regardless of the State of Registry.
The American GPS-for-ADF substitution concession is of no value in Europe.
That said, there is no evidence that any of the countless SR20s and SR22s sold in the UK without an ADF or DME have ever been prosecuted, and everybody has known about this for a long time.