You can cancel at any time until you collect and enter you PIN number.
However, if you’ve paid up front for any extra’s (as you said) these are not part of the Motability contract. If the car has been built the dealer may try and keep that payment, especially if you don’t add the same options to a new order. General consensus is that if the car hasn’t been built and delivered yet the dealer would not suffer any financial loss by cancelling it with the factory so can not keep the payment.
Please excuse spelling/typos. Apart from being a clot it turns out I had one on my cerebellum that's now causing various problems!