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According to PoliticalGaze, Treason was filmed in London and various locations in the neighboring county of Kent. Within London, scenes were shot in Borough Market, Southwark Park, and America Street.
Outside of London, a large portion of shooting took place at Childwickbury Manor which, as the name indicates, is within the hamlet of Childwickbury in the country of Hertfordshire. Childwickbury Manor is also known for being the resting place of legendary film director Stanley Kubrick.
Source: https://www.distractify.com/p/treason-netflix-filming-location
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