(You can also downgrade manually, instructions at the bottom of the page. https://fallout4london.com/release/ There's also instructions on how to manually install the mod if you are having issues with the GOG Installer, there's ways to extract the raw files from GOG installer executables)
Download and Install Fallout London. And select the correct game folder as asked.
???
Profit ! (I'm installing it too at that moment on a Nobara(Fedora) machine. Will tell you if i had to do more than said.)