You will be able to access the United Club in LAX as a *Gold. I did last week and have done so on several occasions before, flying in from LHR and connecting domestically. You should have your (used) boarding pass available to show the agent, together with your connecting *A boarding pass. I always simply say that I'm connecting from an international flight as I hand them over and have never had any problem. It works whether the domestic connecting flight is before or after the international flight, as long as they are on the same ticket.

This is the link in the UA website