I don't think it's down to the 'regional' aspect, it's more due to supply and competition.
LHR is one of, if not THE most competitive long haul airport in the world so therefore SQ has be far more price competitive however good their hard/soft products are. Less supply=higher fares. Ask SRB...
EK at MAN is a prime example, as is SQ. Their prices are higher than they are from any other UK airport, the reason being that there simply isn't enough supply and they are able to get away with charging higher fares.
Simply take a look on EK's website and compare all UK dept points - I will almost guarantee that MAN has the highest fares, certainly in Y anyway...
If Oman, Gulf and, oh I don't know, Cathy, came into MAN, then fares would soon fall. Simple supply and demand.