I’m lucky to get the GT Premium when I did. But as far as I’m aware the alloys are the only exterior difference between GT and Premium, and the alloys are both the same size so it’s just the colour. Premium looks more sporty, GT more elegant. Also, the main differences between the two trim levels (there were only a handful anyway) are taking forever to deliver.
I suspect that’s why they’ve pulled the GT Premium. There must be a lot of angry customers who ordered the Premium just to be contacted by their dealer to say XYZ is no longer available. But you will still be required to pay the upfront cost. Motability don’t reduce advance payments due to missing features. The dealer might offer you some valuable extras for free, like embroidered car mats, a full tank of petrol etc but that’s at their discretion.
Looking back, if I knew thise features were missing I would have just gone for the GT and saved some money.