Is London in England or the UK?

London: Capital of England

London is the capital city of England. England is one of the four constituent countries of the United Kingdom (UK), alongside Scotland, Wales, and Northern Ireland.

So: London ∈ England ⊂ United Kingdom.

It is incorrect to say "London, UK" as if UK were a country-level designation like "Paris, France." The correct format is "London, England" or "London, UK" only in contexts where the sovereign state (UK) is the relevant political entity (e.g., international mail, ISO codes). For geographic/cultural identity: London, England.