Before you log in, your user preferences are not known. Try this:

After logging in, change to a console (e.g., Ctrl-Alt-F1) and log in as root – you'll probably have to use Shift-3 for the # symbol. Having logged in, run dpkg-reconfigure keyboard-configuration again. This shouldn't be necessary, but ... just to be sure.

Now run setupcon to have the your default keyboard settings applied to the console. After that, you should get a # on the console when you press that key; Shift-3 should give you £. Now try rebooting and see what you get at the login screen.

Incidentally, I agree with what's been said – pc105 and gb for a standard UK keyboard.

Thanks for your response. I don't actually want the mini.iso image (nor beowulf – see title).

The netboot linux and initrd.gz are the important files. They are essential for booting into the installer by PXE (iPXE) – this is not the same as netinstall – and (afaik) there are no alternative files that allow this.

This is part of my current iPXE menu, showing the Debian and Devuan install entries for comparison:

:debian
kernel http://debian.proxad.net/debian/dists/stable/main/installer-${archl}/current/images/netboot/debian-installer/${archl}/linux initrd=initrd.gz vga=791 quiet
initrd http://debian.proxad.net/debian/dists/stable/main/installer-${archl}/current/images/netboot/debian-installer/${archl}/initrd.gz
boot || goto failed
goto start

:devuan
set base-url ${httpboot-url}boot/devuan/installer/ascii/${archl}/
kernel ${base-url}linux initrd=initrd.gz vga=791 quiet
initrd ${base-url}initrd.gz
boot || goto failed
goto start

The Devuan files are from the same path in the Devuan tree as the Debian files, but stored locally rather than being called from the mirror at boot. Any suggestions for alternative file targets to make the Devuan installer work are welcome.