Acknowledging presence means to take notice of presence of someone. suppose you are in a party where I am too. we see each other, cross each other but do not exchange greetings or anything. We pretend that we do not exist for each other. that would be called, not acknowledging the presence. In other words, intentional ignoring of presence of someone. The British couple didn't acknowledge her presence, so she was free from the duty of acknowledging the presence of the British couple too i.e. as they don't greet her, she was free from the duty of being polite and greet them back. hope this helps.

These people did not want to engage each other. If one of the two couples involved saw one another, they would have been obligated to have a conversation. One couple did see the other, but pretended not to see. Both couples likely did this. Neither let on that the sighting happened. In this way they could politely not deal with the other.

More like excepting her prescense