Icelandair; now there's a thought.
Good points:
Shorter flights, good fares, departures from Manchester and Glasgow (I believe) as well as London, possibility of an inexpensive overnight in Iceland in one direction.
Disadvantages:
flights east of Iceland in narrow body (757/737) aircraft; west of Iceland can be 767 but not always so check; destinations (from memory) do not include Philadelphia so would have to be JFK or Baltmore.
Might also be worth investigating Aer Lingus. Again, they don't fly to Philadelphia but the transatlantic leg will be on a wide body (A330) and slightly shorter that from the UK and they have connecting flights from all over the UK.
The disadvantage with both these options is the aircraft change and, for my money, unless I particularly wanted to visit Iceland, I would pay extra NOT to have to change planes.