If the Mokka Ultimate is still available to order then why would they be supplying you with a GS Line? You didn’t order a GS Line you ordered an Ultimate so they are trying to change your order completely!
Sounds to me like they’ve sold your car to a private buyer and are trying to pass off a lesser model to you for more money!
At 4 days to delivery means they would have had your original order in the dealership for at least a day or two, enough time to sell it to a private customer, something is very very wrong here!
I would be calling Motability and raising hell!!!