I've just finished a version of this image. I'm still toying with colour toning (do I darken it, make the background cooler in tone and the foreground warmer, etc?), but as it stands I actually quite like the current instantiation. And yet I keep looking at it, not because of the toning, but because I'm looking for tell tale signs that it's a fake.
This image is actually a composite of two images, shown below. I processed both in Capture One and then exported them to Photoshop, where I extracted the girl (Annika) and overlayed her on the image on Mottisfont Abbey.
From these two images, the steps were to place Annika as an unmodified composite image. At this point, I did work dodging and burning to create the shadow beneath her, blending her dress and sharpening the features on her face (image 1). From there I balanced the layers by adjusting individual exposure, rightness, hue, saturation and colour balance (image 2). Then I converted to black and white, added a vignette and did final touch ups where the two images met to blend them (resulting in the image at top).