Translation:This rich businessman from Siberia has bought himself a large house in west London.
I put "This rich businessman from Siberia bought himself a big house in the West of London" which was marked wrong, and the correct answer was given as "This rich businessman from Siberia bought himself a big house in the West End of London." Seems pretty absurd to me. My answer should be accepted.
In relation to all of the above, the West End refers to a specific district in Central London, and so is incorrect, while West London (with a capital W) refers to a specific administrative area in western part of London, which may or may not be correct depending on the intention of the Russian sentence. As a native British English speaker, I would be happy with "in the west of London" and "in the West of London" as acceptable translations.