TDL,
Sorry I can't give you any references, but my information from various journals is that the CAA does not allow single-engine IFR for Public Transport Flights in the UK. There are no similar restrictions for Non-Public Transport S/E IFR flights so private S/E IFR flights are ok.
There are certainly S/E IFR flights in UK airspace and I often fly airways in S/E aircraft.
I would guess its all in the Air Navigation Order, but I'm afraid I don't have time to find you the reference.
QNH 1013