Not sure it's every really possible to produce evidence other than searching the websites. I've just looked at United flights to Newark for the month of May, all but 4 flights are more expensive from MAN then LHR, the 4 flights that aren't are the 4 Sunday flights where flights from LHR are £27 higher. On all other days however direct flights from MAN and BHX are more expensive and at points way more expensive than direct from LHR.
In principle Skip is correct based on a basket of fares and history going back to the old IATA coded zonal system.
Regional fares for long haul were calculated to London with an add-on for a direct regional departure even if sector wise the regional services were shorter !
Now whats further interesting is the direct fares (ex tax) for the basket of fares from London and particularly fully flexible and premium rates are among the highest anywhere !
However on a number of those what could be call railroad routes such as the New York run from LHR we see way way to much capacity in the back of the bus so to speak and with the need to accommodate those fully flexible and premium holders switching rites and the demands for frequency oh and the bribes of rewards lounge access sandwich and tipples (Think i repeated this in another place in this cross forum debate !)- This Y capacity needs managing - Direct sold fares are set a market rate (A margin higher than regionals in some instances) whilst a significant portion of the capacity is dumped and sold via Agents and those Website comparison sites( Who make money via click through )
Its also a case those dumped tickets are priced such that you make own way to LHR
The thing is the xyz-LHR-JFK (or similar) pricing really is designed to favour just about anywhere other than the domestic market . xyz could be say Rome where premium fares via LHR are frequently many hundred less the offered for those of us in the UK !
Its all designed to support the Hub concept but is it fair especially to the UK customer probably not.
And does it effect to viability of regional long haul certainly.