This is the Motability Guidance which basically say’s it is up to the dealer how you pay the AP.
How you pay your Advance Payment depends on your dealer.
Bank Transfer – every dealer accepts this, but you are likely to have to send the payment one or two days before you get your vehicle.
Debit card – some dealer’s accept up to £1,000 this way, but not every dealer does.
Credit card – only a few dealers accept this.
Cheque – dealers do not accept this.
Cash – most dealers do not accept this, but this is down to each dealer’s discretion.
So it looks like our opinion as the customers does not seem to matter.