I have also replied to your within our UCL Moodle Users forum, but am posting my reply on here too for the benefit of others outside UCL.
You can download the annotated PDF file, however, on our current UCL Moodle theme, this column is obstructed by the blocks on the right. In the new theme coming over summer you will be able to hide the blocks with one click and make the page full-screen, so this won't be an issue any more. Hopefully other Moodle installations won't have the same issue with the blocks covering the columns, like we do.
For those who can't see all the Assignment columns because the blocks are covering them, if you zoom out (Ctrl--) you can see the 'Annotate PDF' column. If you click on the first link for each student the online marked PDF file with its annotations will open.
There doesn't seem to be a way to download these in bulk from Moodle, but you can use a browser plugin to do this pretty easily. Chrono Download Manager looks like a popular option for Chrome Browser. You can filter by document (on the second tab) and then filter again by PDF file. I think it will also download any PDFs that students have submitted (i.e. the non-annotated files), but hopefully these are easy to sort by filename, once downloaded.
I hope others find this useful too.