I've always thought Prunella and Pat are a couple.
It's interesting and unusual actually, how there are no main character children in the book or film. Prunella and Pat seem to be happily childfree, whereas Fred looks like he's single especially when we see his flat in the film.
The cat family is the main point, and they are a mummy/daddy/children leaving home traditional kind of setup, so Fred, Prunella and Pat in the background are a nice nod towards the fact that different kinds of happy adult lives are possible too.
I think there being no children also probably allows the children watching or reading to identify with the cats more, and they are the main characters so it makes the story more immediate.
Have now watched/read it 4 times in the last 24 hours and am overanalysing