You could say "and they went off to the shops." but it is clumsy because you have already mentioned the shops. You could add more description, such as "and they went off happily chatting about the film." Or "Meg said "No, I hate shopping." And went off in a huff. You couldn't just say "and they went off." Food goes off (rotten) if it's left out too long.

This kind of word order change is a stylistic technique used in English-language literature to make a phrase express a slightly different nuance. (Here, I think "and off they went" expresses the nuance that as soon as "yes" had been said, Jill and Meg didn't spend any more time on thinking, they headed for the shops without delay.) It's creative writing that adds more feeling to the text and makes it more pleasant for the reader.