It's the fun of paying the YQ surcharge. Plus, if you fly through the UK as a stopover you get to experience the joy of Air Passenger Duty.
You won't have access to a lounge in PHL, but you will have access on your return. This is because, according to the rules, the return will be a connection on the same day, while your outbound won't be:
Connecting between oneworld marketed and operated flights:
First and Business Class customers connecting on the same day of travel, or before 6am the following day, can access the lounge when travelling between an international long haul (a oneworld international long haul flight is defined as an international flight marketed and operated by any oneworld carrier with a scheduled flight time longer than 5 hours) and an international short haul or domestic flight (and vice-versa).
Lounge access will be determined on the international long haul ticketed flight (either First of Business Class) regardless of the ticketed class of travel on the international short haul or domestic flight.
You must be prepared to show your boarding pass or itinerary showing travel in First or Business class on the international long haul flight, in order to access the lounge before your international short haul or domestic flight.
You may find a sympathetic agent, however.