There used to be a left luggage facility at IAD but it was closed after 9/11. Can you rent a car and put everything in the trunk? I heard the Udvar-Hazy center has medium sized lockers for luggage so that may be an option (call them to make sure). My guess is you may not be able to check your hold luggage all the way through as I think you may have to claim it at customs. After you depart customs you'll have to check it with BA but I don't think their ticket counter is open when the NH flight arrives in the morning. The NH flight arrives during a slow period at IAD for intl. arrivals so hopefully it won't take too long for immigration/customs.