Interesting that, even if we take off airline limitations and look at any airline...
LHR-EWR-LHR ... cheapest £439 in september
EWR-LHR-EWR .... cheapest £647 in september
I think its a simple case of Supply and Demand.... if you look at fares in October, for example, the spread narrows (e.g. there are a few for £425 EWR-LHR-EWR).
I suspect its down to the fact that its September... shoulder period between the summer silly season and off-peak. September's probably still quite a busy time for the airlines.
Your best bet is to do a one-stop via the mainland (i.e via CDG, AMS or FRA). I also know people who take Icelandair via KEF... that's quite cheap apparently !