Never flew IAD-LHR, but once flew (on BA) from JFK-LHR daytime to get in position for an LHR-JFK Concorde return the next day.

It was an excellent experience - stayed awake on the flight, arrived LHR in the evening, went to the Hilton at Terminal Four for a great night's sleep, and was pretty much fine the next day. Definitely less jet lag than on typical evening flights from Eastern North America to Europe.

Give it a try!