sqlrob: How do you install this without using Galaxy? Is it just a matter of running the backup installers for both?
I just installed Fallout 4 with the off-line installers. High Resolution pack is not needed. I started the game up and configured it in the launcher once, just to generate the configuration files (I don't know if that's needed, but why not).
After that I did a off-line install of the London mod, and it installed without any problem.
After launching the mod it asked for the Fallout 4 directory (it found mine without any problem). A small hint - If you have not installed the game on the standard C drive location, do NOT try to use the map icon, because I heard it's buggy. Just type in (or copy) the full path in the space. The mod them starts copying (or moving - I don't know) files from the Fallout 4 directory to the Fallout London directory.
After all is done, you can start the mod. It takes a wile to start up, but after that everything runs smooth.
So - Yes. You can install the mod without using Galaxy...