My spin on this is that you would probably be allowed admission in PHL if you show the onwards boarding passes, the staff try to be helpful, but I don't think you have the right to it. The PHL lounge isn't anything special, however. LHR outbound no problem. Inbound, I think you'd be unlikely to gain admiission airside, though of course you could use the Arrivals lounge if the timings allow it and you have immigration rights to the UK (Arrivals closes at 14 hrs, and there's no much point going in after 13:30 hrs).