Unfortunately, it's a little hard to say from a single, distant still image. Without any film to see how the animal moved, it could just as easily be a sheep dog.
That being so...I can't imagine anyone out on a hike saying to themselves: "Oh! There's a sheepdog! I know, I'll take a picture of it and tell people it's a big cat!"
I do believe that the evidence of big cats in the UK is so utterly overwhelming that there can't be any reasonable doubt that they are out there, so there's no reason to believe the photo isn't genuine.