How can I get the cheapest flights from Washington to London this summer?

Can you, on BA, skip the outbound leg of a return journey and only use the returm, if you told them that at the time of checkin? Because a return flight is significantly cheaper than a oneway flight.

I'm putting this on the BA thread because that was the cheapest return option.

Thanks for all help, replies etc.

Details:

Propesed journey:
KIAD -> EGLL
Nonstop preferably
1 adult
27/08/2014
Preferably the 20:45 departure.