When you say "non-linked" do you mean the ORF-IAD and IAD-LHR portions are on different tickets?

Even if that is the case, I'm sure that UA and VS have a interlining agreement. Unless you need your luggage for some reason in Washington, when you check in at ORF, I'd ask the UA agent to tag your luggage all the way through onto LHR. I've never had any problem with UA doing it for me - the only problem I've had is the UA agent trying to figure out HOW to do it.