Yes, LAX, DFW and MIA all bring the price up considerably (approx 300USD). I am quite flexible with dates. The plain LHR-PHL via ORD gets priced at approx 600USD.
I was wondering whether there are some known airports that one can normally sandwich in on the way from ORD to PHL perhaps south to backtrack north again but still flying west to east. Cheapest I have found so far is to add RDU (LHR-ORD-RDU-MIA-PHL) but it still brings the price up by over 150USD.