they can ask for a deposit but the ap doesnt have to be paid in full.this is what the motability website says:
When you pay it
The amount is payable in one lump sum to your Motability Scheme dealer either before or on the day you collect your new car. Some dealers do ask for a holding deposit when you place your order, but any deposit paid will be deducted from the Advance Payment you pay when collecting your car. If you want to pay by cheque, this must be given to your dealer at least seven days before you collect your car, to allow funds to clear.