(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.
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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.)
Thanks for the game! Now, I started Fallout 3 not long ago, is there any commonality to the story between the Fallout games? Are they following each other? Are they alternatives of the same story? Is it something else?
You can probably play the games in any order, but beware the first two are rather classics today, within the technical limitations of their time.
There's also the Tales of two Wastelands mod, which combines FO3 and New Vegas on the latter's engine. I've installed that, but admittedly was too lazy for a complete playthrough.