Fallout London disables Fallout 4 GOTY. If you want to continue or replay FF4GOTY, you'll have to delete F4GOTY AND Fallout London, then reinstall F4GOTY

Also handing over your first born is required.

Don't forget to appease the wizards!

Thats for skyrim ya silly.

It's almost like they explicitly tell you this right on the store page.

Don't mind Keith. In his own words hes older than Jesus, singlehandedly prevented worldwide communist revolution in Vietnam (and won that war), and can out bench press a steroid infused freak 60 years his junior - so ya know...

Well, someone doesn't read the installer notices do they!

So, yeah we already know, it literally tells you that an reinstall will be needed to play FO4GOTY edition when you install or update the mod.

Use a mod manager. If your FO4 is from GoG, you can simply make a new profile in the mod manager for Fallout:London allowing you to go back and forth as you please.

Vortex or MO2. Vortex may be easier for beginners. Gopher has youtube vids on installing FOLON and covers this within the video.