Having flown this now I thought I'd update with my findings.

U Class on a domestic 777 service does indeed book into the 3-class First cabin and I was assigned suite 1L for the 2 hour flight.

F Class books into the Business cabin.

Not sure if seevice was different but there was only one other pax in the First cabin and we had our own FAs, were given slippers, had menus for meals with many choices, including the daily special which I'd seen in long-haul reviews.

Tbh felt like what I'd expect long-haul First on MU to be like.

Note that U Class wasn't for sale on the website, I believe it has to be booked over the phone or via a TA.

Expertflyer lists U as Economy, this is definitely incorrect on domestic 777 services.