I'm using a GoG version of fallout 4, with the default Fallout London GOG installation via the galaxy launcher.
It's as 'bog-standard' an experience as I can find.
So why is it immediately broken?

I hit 'play', I get the Fallout London main menu, Fallout London opening cinematic, then I get Nate and Nora talking about the veteran's ball.

I tried to hit 'continue' anyway just to see where it took me, but it looked like an endless loading screen.