I do get your point. However, on testing this getting up slowly is impossible whilst keeping heels down(unless I throw my weight forwards to get past a 'balance point', and super easy if I let my heels come off the ground. I don't think you'd actually notice this watching someone unless actively looking for it.
Sorry for wasting your time, I should have seen that as obvious. I've now obtained a set of lifting shoes and if I bend to as close a horizontal back as I can manage I can get down to parallel without a plank for my heels when doing a bodyweight squat. Now I just need to work out how to do that with weight on my back so plenty of testing and trying different things ahead as I'm now struggling with my shoulders and getting the bar far enough back. Normal grip I can't get back far enough(probably half my problem), wide grip I feel as if the weight is going to slip away. I'm gonna carry on with what I now realise were more or less high bar squats whilst attempting to sort out low bar form over the next few sessions.
"Unless I throw my weight forward"
That's why the back mustn't stay vertical. It's necessary to move the CofG over the middle of the foot, which is the point of balance. The weight must stay over the centre of the foot using the angle of the back. I suspect, though I have never confirmed it, that lifting the heels is a result of a back angle being insufficient, hence the need to pivot forward onto the toes, which isn't optimal for stability/force production.