You guys beat me to it. Gog has it for free, so I added it to my library. I have not tried downloading it yet, as it is quite large.
I also noted that it rolls back your game version of Fallout 4 to an earlier version. If you have the Gog version that is easy, the London installer will do it for you automatically. If you have the Steam version that looks like quite a lot of acrobatics.
The deets on how to do it are here. If you have the Epic version, it is just impossible to play F London.
That bothers me, because does this mean we also have to find older versions of F4SE? And older versions of every mod that requires F4SE?
I currently have the Steam version of FO4 installed. But I did buy the Gog version last year. I just never got around to installing it. So maybe sometime in the future when I have a lot of time I will archive my Steam version, install the Gog version, and then try Fallout London on top of that.