Miss Scarlet & the Duke’s production never set foot in London. The first two seasons of the show were set in Dublin, Ireland, using old Georgian houses and architecture in Ireland’s capital, Dublin, which hasn’t changed as drastically from the show’s time period as the architecture in England. The street where Eliza Scarlet’s character lives is actually Mount Pleasant Square, Ranelagh, Dublin. The show used CGI to cover up parts of the sky and make it appear as though classic British architecture was visible in the distance. The third and fourth seasons of the show were filmed in Belgrade, Serbia.

Source: https://www.distractify.com/p/where-was-miss-scarlet-and-the-duke-filmed