Don't get what they were thinking, that Andrew St Pierre -White (speeling) chap did a video a few weeks ago about that very issue, it seems if the batteries do go flat it's not just a case of grabbing a handy jump start it needs the correct software via laptop to release the park brake once started, presumably it doesn't re-learn.Just be glad you dont own a new defender wot powers the computers on each time you look at it...
One would hope there's an emergency release somewhere, and it might be possible to remove the park brake motors from the rear calipers as a get you going measure.
Either way far too complicated (and costly) for me.