In the U.S., you could standby for an earlier flight for free. Not sure if you can do that on UA from YOW.

You can research other late flights from IAD to Europe on UA and LH so that your're prepared in case of missing your flight. Good to kinow your options. I would also include flights leaving around the same time (in case they are delayed then they become options). But I don't think there will be many options with such a late departure time. Then on the next morning, you can connect from FRA (or whatever) to LHR.