To answer your ?. The advance payment will be price when you ordered the vehicle. Direct from motability website following quote..
Our price guarantee
Our price lists are usually updated every three months, so you can be assured you’re getting the best prices from each manufacturer. New prices are usually published at the start of January, April, July and October.
The Advance Payment on a car can change throughout the year. We operate a price guarantee, which means the Advance Payment of the car on the date your order is accepted won’t change, even if it changes while you are waiting for the delivery of your car. The only case where the price may be altered is if the car description is changed subsequent to the original order.
https://www.motability.co.uk/about/how-the-scheme-works/payment-and-cost-for-cars/
Cars On Motability
2009 Citroen C4 Grand Picasso
2012 Vauxhall Antara
2013 Skoda Superb Estate
2016 Seat Alhambra
2020 Seat Tarraco
2021 Ford Kuga
2024 Nissan X-Trail left in Sep 25