Big Sur 11.2 - The update could not be verified

I get a "The update could not be verified" error when trying to update to Big Sur. This happens just after the download completes. This is happening on a clean install of Mojave (10.14.6) on a Mac mini (2018), 64GB memory, 2TB Disk, with just required security updates installed. I've tried safe mode, an new user with no link to iCloud, WIFI, ethernet, broadband in another house firewall up and down, resetting PRAM, toggling 'set date and time automatically', and clearing out the contents of Library/Updates. The log is below.

I would really appreciate some help - it's hard to move forward when I can't even download the update.

There's some back story to this - I couldn't install 11.1 either! I could download that version, but every time the install got to 13 minutes remaining it failed with "An error occurred loading the update", with a log error:

init_set_autoaddr(en6,1) failed, Operation not permitted (1)

The same error doing an update, or clean install from bootable USB, wifi, ethernet, safe mode, PRAM reset.

During this week long upgrade adventure I've reinstalled Mojave several times via Recovery - that downloads and installs without any problem.