I have seen a few posts wanting to know when the Now TV output will be upgraded from 720P to 1080P, so I contacted Now TV support today to get an update.
They confirmed that this is definitely being 'worked on' but they could not confirm when or if it will be available. They also could not confirm that it will arrive in 2019.
It sounds like it is not a quick & easy thing to do, otherwise we would have it already.
My personal view is that we will get it eventually as there will be a strong commercial reason for it, because competitors are commonly providing 1080P or even 4K output now.
Hope that helps some of you.