How much time should we allow between landing at Heathrow and catching the Eurostar? I need to make both reservations. I would expect that we arrive in the early morning.
I would allow a min of three to four hours. IF you plane is on time, but I would allow six to be comfortable. An hour to off the plane, clear immigrations & customs, 15 mins to walk to the tube, and an hour and fifteen on the tube to the Eurostar station. And you need to check in at least 30 minutes early - that ate up 3 hours with everything clicking on schedule. If your plane is a little late, could be closer to four. If something else goes wrong, who knows?
Is there some particular reason that you don't want to book plane tickets all the way from Philadelphia to Paris? I love trains, including the Eurostar; but I suggest that if you can't book a direct flight to Paris and have to arrive at Heathrow, then book a fairly cheap ticket for Heathrow-Paris on British Airways. You'll save time and the tickets may not cost much more than the best Eurostar discount fare tickets.
Tim raises an excellent point. Once at the airport, stay at the airport. Better yet book an open jaw ticket - into Paris, home from London.
We did the Heathrow - Eurostar to Paris arrangement because we wanted to see what it was like to travel on the Eurostar. We allowed about six hours to get from LHR to St. Pancras so we had a "cushion" if the plane was late. Since it wasn't, we had an enjoyable visit to the British Library treasures room (very near St. Pancras), then a pleasant lunch in St. Pancras station.....It was great arriving in the center of Paris instead of a sterile airport.