I agree, being able to preview in full screen is very handy.
I have written a small utility to do just that.
https://afp-fullscreenpreview.sourceforge.io
Short Demo: https://youtu.be/h7YzDgH0DMg
It's an AutoHotkey script. It's pretty simple. In essence, it clicks Photo's 'Copy Flattened',
then launches IrfanView in full screen mode as a clipboard viewer.
PREREQUISITES: https://www.autohotkey.com/ (get the 'Current Version', v1.1.x) https://www.irfanview.com/ (get the 32bit version)
INSTALLATION:
After installing the prerequisites above, extract all files into a new folder somewhere. Click on the script file (AFP_FullScreenPreview.ahk) and a new icon should appear in the System Tray. A shortcut will be created in your Startup folder so the script will start automatically each time you log on. This behavior can be turned off in the .INI file.
USAGE:
With Affinity Photo in the foreground, in the Edit persona with an image open, pressing the hotkey (F11) should 'Copy Flattened' the current selection (or the whole image if there is no current selection) and then launch IrfanView in fullscreen mode with the preview.