You don't actually have to use the return ticket back to the states if you don't want to, so if it's cheaper, I'd go for it. I'm supposed to be moving to the states in January and I'm already planning on buying a return ticket, even though it's unlikely that I'll be able to visit the UK for several months after I leave - it'll be much cheaper than a single fare!
The return ticket gives you the option to fly back to the states in December if you wish, or you can just forfeit the flight and stay in London instead.