Middle grade is largely a USA term. Don't know about Australia. Perhaps UK publishers are drinking US Kool Aid. It's Publisher Jargon.
I've spoken to Librarians and Bookshop owners, being involved with Publishing and Writing. They seem to think Publishers are pretty clueless. Look at how many paper books make advance and the TINY proportion that are successful. They might as well put a list of submissions up and throw darts. They make the Pop industry look expert.
The big ones won't take new authors in the the most popular genres (Romance, Thriller, Detective in broad terms) because they can get all they want from the established names, some of which are actually "factories" of ghost writers (the
Patterson brand titles) like the
Stratemeyer syndicate of old.
I was shocked to read that Penguin's Puffin Imprint for many years only reprinted known winners as paperback. Stig of the Dump was practically their first "original" title. They were so "proud" of that. Yet he was an existing author.
Really the Publishing industry is sick and has been for decades. No wonder Amazon is taking them to the cleaners and aims to cut them out entirely by being a publisher, retailer and library. Paper, etext and audio.