I own F4 on Steam, but I was unable to downgrade it to play London. So I decided to buy F4 again on GOG just to play London and I am not dissapointed! The mod/game is great! It needs Buffout 4 to play without crashes, but other than that I had no issues. 7 hours in and I am excited to go through all of this. I will donate to the developers for sure, this is what Fallout fans needed for a long time.
I own F4 on Steam, but I was unable to downgrade it to play Fallout London. So I had to buy F4 again just to play London. I would rather give my money to Team FOLON than Bethesda. Anyway - Fallout London is great so far (6 hours in), so I can recomend it to others.