Quoting Navpi: -
Not sure if representative but interestingly BA fare LHR JFK is over £132 cheaper from LHR than same dates 1st week July than EI Ex Manchester to JFK.
Is that to make up for lack of premium ?
It seems a nice hike .
It used to be said that because airlines flying across the pond from LHR, (I'm thinking BA in particular), were able to attract plenty of people up front at premium fares, they were happy to discount economy prices to help fill up seats at the back. Whether that was direct by the airlines or through other booking agents, and whether it still holds, I don't know. Maybe Skip can tell us.
Of course, if demand is strong on a particular day from MAN, that will be reflected in the EI price.