We are within our rights to shoot a dog which is worrying our sheep. And worrying can mean just running about off lead, even if it's not actually chasing the sheep.
However, firstly, many of us won't be a good enough shot to do this effectively.
And secondly, most of the farmers and sheepkeepers I know, however angry they are at damage to their sheep, also love dogs. Many couldn't shoot a dog, and many would suffer at least as much stress from having had to shoot a dog as they would from damage to the sheep.
It's something that never seems to get mentioned.