If Fallout was set anywhere else but America, expect massive changes, not just in geography, but the entire setting. FOr one, there are no Vaults in Europe, or Russia, or Australia, or whereever it could be set outside the US. They might have some bomb shelter system, but nothing like the Vaults, well except Russia, since their whole metro system was actually designed to act as a nuclear fallout shelter but we already have a post-apocalyptic game franchise for russia, so that one is out anyway.
So, Vaults are gone. FEV is gone, which means no supermutants or similarly mutated critters such as deathclaws. The thing about the mutated critters is that during the War several bombs hit West-Tek which released FEV into the atmosphere which sort of bonded with the radiation and settled on the surface, i.e. ridiculous mutations. Ghouls are unlikely since their creation requires a sudden, massive burst of radiation without the heat and pressure that a nuclear blast produces, given we lack vaults..unlikely.
Now onto the Setting. The war happened in 2077, not the 1950s. America was culturally stuck in the mindset of that timeperiod, they basicly never had their 60s, 70s or 90s. Hence why for example they have those oversized cars and those 50s fashion. That does not mean the rest of the world was similarly stuck. Say China, or Japan, or France could have had, culturally speaking, a highly advanced society, post-modern so to speak.
So to recap:
-No Vaults
-No FEV (Supermutants, Deathclaws, probably Ghouls)
-No 50s stuff
And thats just the changes which are fact at this point. Basicly no country other than the US would function like the US does if we would set the Fallout games there. The games would be so radically different that they may as well make it a new franchise entirely. Plus all your known factions are gone. All of them without exception. Because their existance is based upon very specific and precise circumstances that you cant just re-create elsewhere. Fallout: London would NOT be Fallout. It is that simple.