This is most odd! I know (from my FAA PPL/IR) about MELs for N-registered aircraft. I even know where to find the MEL for the ones I've flown in the US.
However, there is NOTHING in the POH for our G-reg Arrow (PA28R201) that refers to an MEL or even mentions a compass. The CAA C of A doesn't mention one, either.
So, nothing in anything I have as the "legal" aircraft documents says I need a compass.
I have a copy of the UK AIM, but the index is nbg and life's too short to read the whole book looking for CAA mandating of a compass for flight in the UK, or mandating the FAA MEL. However, bookworm is famous for knowing his way round that tome, and if he says it ain't there, that'll do me.
While I would never fly without a compass (other maybe than in the circuit, where I can see what's going on), I'm baffled as to how I'm supposed to know a compass is mandatory. If it's in some secret document, we need to take that up with the CAA to avoid folks being prosecuted for failing to comply with laws and regulations they know nowt about.