Enjoyable but flawed.
First and foremost, a trigger warning: this game features dead cats.
Fallout London has a lot of good things going on for it not least of which is that its story, without going into spoilery details, is much better than the Fallout 4 one - I know it's not a very high bar to pass but this is not sarcasm.
London, in all its overgrown glory, is a beautiful place to explore but this is where the game hits a hitch: many roads and alleys are blocked off which results in exploration and quests taking far longer than expected because the player needs to fight their way through several miles/kilometers of hostile ground.
Still, the city looks and feels lived (and died) in, there are surprisingly many factions, plenty of quests ranging from mundane to marvellous and there tends to be environmental clues to warn you if you're approaching a particularly dangerous place so that you can either avoid them or go in and give them what for.
The worst sin that I feel the game commits, however, comes right at the end where the player is forced into doing certain things that are both incredibly stupid and not explained in any fashion. It's this thing that led me to giving Fallout London only two stars as it ruined all the enjoyment I'd built up going through the storyline.
Your mileage may vary, of course.