I own Fallout 4 on GOG and Steam.
I uninstalled modded Steam F4.
Installed F4 GOTY through GOG Galaxy.
Run F4 once to create settings.
Installed Fallout London through Galaxy.

When I press play it throws up Fallout 4 installer
(it points to C:\Games\Fallout 4 GOTY\ folder) , however when I press Install the installer just exits?

WTF am I doing wrong????

I had a similar problem (downloaded through GOG but also had a Steam version)

Try going to C:/Users/[Your Computer Name]/Documents/My Games/Fallout 4 (or wherever your old Fallout4Prefs file is that Steam created) and renaming your Fallout4Prefs file to something else (Fallout4PrefsOLD or whatever).

I'm on Windows 10 so maybe your path to the Prefs file is different.

After I did that the Fallout London installer worked and made its own new Fallout4Prefs file in the same location and the game ran.

I managed to install Fallout London by disabling firewall and antivirus.