I believe LHR based flight attendants only need permission to work in the US in order to work a IAD-LAX flight. I don't understand why you think UA should deadhead them when they are eligible to work the flight. There are many LHR based flight attendants who are eligible to work in the US. There are quite a few who are US citizens. Since LHR FAs are also AFA, I fairly sure there is no union rule prohibiting such work. I doubt UA would ever risk having non-eligible crew work such a flight.