I know everyone goes on about how bad the London airports are, and if you are looking at flights from Manchester I assume you live "up North". However, connections over US hubs can be a bit of a nightmare. I'd seriously consider coming down to London and flying non-stop (which means either BA or Virgin). You could say that transatlantic carriers are in a race for the bottom; the US carriers (all of them) are leading that race right now.
Inbound connections at US hubs mean queuing for immigration, collecting your bags, going through customs, finding check in to hand over your bags again (usually not a major issue - just outside customs), find you way to the next terminal, go through US security (don't buy any duty free liquids or if you do make sure you put it in your luggage before you recheck it) and then find the gate.
Coming home is easier (hence the shorter connecting time) since you don't have to do customs and immigration.
A thought - the fare will almost certainly permit a stop of up to 24 hours at the point where you change planes so you could actually stop in Philadelphia for a night either/each way (at your expense). I did that once coming home from Denver via Chicago and, by getting a flight out of Denver that arrived Chicago after the London flight had departed I got the best part of a day in Chicago.
But in the end I strongly suggest either driving or taking the train to London or even take a connecting flight to London.