@Dougall1248, Sorry, I edited my post after you responded.
It doesn’t heat the cabin. It only heats the battery as a natural by-product of charging (that’s what the ‘Keep battery warm’ option does by whacking in 30 mins of charge just before your target charge time unless you have the ‘Only charge off-peak?’ option set).
@Medved_77 explained how to achieve what you want more fully than me.
I don’t think the ‘Keep battery warm’ feature is actually very useful. What you really need to do is turn the climate control on. I have a Siri Shortcut to do that. This means that I can shout at my phone in the dark about 45 minutes before my scheduled departure time to get the battery and cabin warm or tap on the screen if my bleary eyes have cleared by then. Other respondents have suggested this doesn’t improve efficiency, but I disagree. The only modification I shall have to make this winter is figure out how to stop ev.energy from disabling charging whilst this preheat occurs to mitigate the battery loss. The car can draw up to 13 or 14kW during that preheating.
As I wrote earlier, I can completely understand why you might wish to only charge during that four-hour window, but that would rarely be enough charging for my needs, particularly in winter.
Anyway, hope you figure out how to achieve what you want.