First off, there isn't a true horizon, so to say it's sloping is incorrect. Looking at the uprights of the building it is in fact level, so it's pretty obvious the hills are sloping naturally (as you'd expect really). However, many of the clouds are blown and it is a good example of where a neutral density graduated filter is needed, although if you shot in RAW, you may be able to recover a fair amount of detail with a grad filter in Lightroom. That still won't change the harsh lighting though and as much as possible, landscapes are best photographed in less harsh lighting, either under cloudy skies (where it may be difficult to get impact, depending on the type of clouds) or close to sunrise/sunset, when you have natural diffusion from the greater amounts of atmosphere that the sunlight travels through when the sun is low in the sky. Compositionally, the buidling would be better in the bottom left hand third. Having the main subject is usually (but not always) more pleasing to the eye, but in this case, it would also remolve the car park which isn't adding to the image and remove some of the grass, with too much blank space. This would also add dominance to the sign, which with the angle, could act as a leading line and of course is part of the story. Also, I'm not sure that a wide-angle is the best option here, due to the distortion and lengthening of the sign, so if you do get a chance in the future, try a standard focal length from a slightly different spot.