Mastodawn

Download and Install Fallout 4, from Steam or GOG
Downgrade the game using their own Downgrader tool www.nexusmods.com/fallout4london/mods/1 Source and latest binaries here : github.com/Fallout-London/FOLON-FO4Downgrader A bit in panic at the moment, you may need to compile it yourself to fix some openssl version issues or to make it launch on Wayland. Consult the Issues tab on GitHub for common fixes.
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.)

Fallout London Downgrader and Addons

This mod includes the Fallout London Downgrader, which allows you to downgrade Fallout 4 with a few button clicks. It also includes various goodies that are required for modding Fallout London such as

Nexus Mods :: Fallout 4 London

[Request] Full guides to install GOG and Steam versions Fallout: London for Steam Deck - sh.itjust.works

Fallout London Downgrader and Addons