I've relatively often used an outward portion over two days, and although I always expect it to be questioned, it hardly ever is. This has tended to be on longish journeys, from the N of Scotland to London, but not ones which would be impossible to complete within a calendar day.
My interpretation of 'not able to complete' has always been that it applies to a journey starting at the time that I choose. Therefore if I choose to start out from Inverness at 7pm on my way to London, it seems reasonable to expect that I can overnight in Glasgow and continue the next morning, as it would not be possible for me to get to London that night. And there could be perfectly reasonable grounds for wanting to start at 7pm, or not being able to start before then.