The novella 'What Abigail Did That Summer' is pretty explicit about the location of The Folly.
...sits on the south side of Russell Square on the corner of Bedford Place.
As the west side of Bedford Place contains a glass and metal office block that would make this The Folly.
50 Russell Sq https://maps.app.goo.gl/XdyAU5nYcXoXgqfM6
However... this doesn't really match the images in the graphic novels which show a wider double door as the front entrance unfortunately.