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.

The penalty will be against the original selling dealer - there's little they can do to penalise the customer.

Pretty desperate tactics from LandRover dealers, who are themselves desperate for stock to sell.

This can only come into play if the car is owned outright, at which point the owner can sell it as they wish. Land Rover has no say over another's owned property. On financed cars, it could be enforced in some way, but even then, seems like they're trying their luck.

6 months is no time anyway. Just sell it 1 day after the 6 months 'deadline'.