This has been discussed in so many threads here.
Dogs “can” be dangerous. I didn’t see where it said the dogs were tied up in the garden? May have missed it.
However the garden should be secure.
The animals should be well cared for. As the OP said he has given them information to help, which may not go down well, but any owner should be aware of specific welfare for their dogs.
The issue here is the dog escaped. If the attack was in the dogs garden then I would say fair play, its in the dogs territory, dog chases cat etc, God forbid it caught the cat which would normally be straight out over a fence.
The RSPCA would be able to help in assessing if he dogs are well cared for and help or educate the owners.
I think it would be a police matter as the dogs were not in a secure environment and attcked an animal, realistically that should not happen.
Its also down to who maintains the fence etc I guess, but if you have a dog wether it is or is not your fence you stil have to make that provision.