Hi!
I have a RED 15 without WiFi module.
(Was built in 2017 - Rev. 1)
Unfortunately, interrupting the boot process does not work.
I am connected to the console cable and have let the box restart itself several times, but it always boots into SophosOS.
Either something has been patched or the RED15 is immune to boot interruption.
I can only start the failsafe mode. 
Update here