Here's a couple of ideas. Try the European version of Orbitz, etc. called Opodo. You'll often find fares not found here in the US but still in and out of the US. It's called opodo.co.uk

The other is to contact London bucket shops and get fares based in other currencies that the US dollar is strong against. Here's a few to try: www.netflights.com and www.cheaptickets.co.uk and www.allisonbrothers.com and www.citibond.co.uk

hope this helps a little.