Originally Posted by LHR/MEL/Europe FF

In theory, anywhere that LH F operates. But only at T-14 at the earliest, and even that is sporadic. Often it's much closer in, like just a few days. Also, LM doesn't show all LH F, blocking random days... so it might be available via UA on 2, 3, and 4 days before departure, but LM might only show day 3, blocking days 2 and 4.