The reason dogs go for cyclists and postie's is because it's a dogs natural instinct to protect it's property. When the postie comes, or the cylist rides past, the dogs bark to "see you off". They don't realise that postie's walk away anyway, and cyclists cycle past anyway, they think it's because they've seen you off. That's why they react different to cyclists and postie's than they do to normal visitors (because it doesn't work with normal visitors). It's reinforced behaviour.
Collie's won't maul like a pit bull/bull terrier type dog. bull type breed of dogs are bred to hold on and able to lock their jaws. That is why they do so much damage to children when they bite, because they don't snap like other dogs, they hold on and shake their heads. However, all dogs of whatever breed are capable of a nip or bite given the right circumstances.
I think maybe it's because you surprised the owner, and the dog sensed her surprise and reacted to you. I think the most sensible thing to do on this occasion is to go back to the woman (presuming you know who she is), show her your bruise and tell her she needs to muzzle her dog in public because, if you can surprise, then so can someone else, and it could be a child next time. If she's argumentative or doesn't care, then i would report it to the police and ask them to have words, but I think demanding the dog be destroyed on this occasion is a little bit over the top, as the dog was probably just protecting her.