It depends on what the order said. If you have already bought and paid for a car under one set of terms, they can't force you to sign something else to receive the goods.
I suspect dealers are being told to include the forms at order stage (because nobody reads everything) and are being encouraged to push them at handovers.
As for stopping speculation, it sounds a lot more like trying to get the profit on used cars instead of letting someone else. If they are allowed to do it, what's to stop them throttle production to drive up used prices whenever they need a few quid.
If I was asked to sign such a form, I might ask the dealer how much they were going to pay me or if they wanted to split the profit on resale.
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