Get the Lounge Buddy app for your phone. When you register the app, you go through the long list of cards that give you different lounge access, it then tells you which lounges you can use, based on where you are.
Hint - the US American Express is waaaay down the list, beginning with "platinum", not "american express platinum".
Also go into Lounge memberships and select "Priority Pass Select" after you register for it through the amex site.
I'm in San Diego, so I only get to use the lounge in T2, but that's across from the only international gate. I can go through security early (BA and JAL don't offer PreCheck), hang out in the lounge, and walk across to the gate when it's time.
I did get to use the new lounge in Heathrow last month. It was SO QUIET compared to the main concourse.
Also, if you are American, get Global Entry, which can (after a certain period of time, and a certain number of UK entries) will let you apply for preferred entry into the UK. My co-workers were 2 hours in immigration, I was through in under a minute. I was at my hotel in London before they got their bags.