Other than getting the police to prosecute the owners if the circumstances justify it, I think you have covered all bases.
Dogs can only be shot 'in the act' ie where its essential to prevent further killings, but a judge could order destruction or more likely muzzling and on lead when out in public. Not sure I would want that for a first offence since its the owners fault not the dog's, but it might be worthwhile pursuing the muzzle/lead order.
Im so sorry you have gone through this. We lost a young sheep a couple of years back, with another ewe injured, but didnt see the dog, but the injuries were classic dog; the following year three lambs were killed nearby and the dog was seen this time, so the description is known by the village even tho it couldnt be caught to prove it was the specific dog everyone knew it was - but they are on a general silent warning from the village now! One good thing is in your case the dogs were caught and identified.