Are you overnighting in London, or connecting the same day as you arrive? I think you're staying overnight. If you check the bag through from the USA, the piece system (70lbs/bag) will apply, but if you have to claim them at LHR, then re-check the following day, AND have a separate ticket for that flight, chances are you'll be on the "rest of the world" system of 20 kilos or 44lbs.