No deposits are required for cars when ordering on the scheme, any deposit asked for by a dealer is for them and has nothing to do with Motability.
I have never paid a deposit and when I have been asked for one I refused, only once did I have to go to a different dealer as they wouldn’t accept an order without one and insisted it was non refundable so I walked away.
If you want to risk giving a deposit then that is between you and the dealer and may be difficult to get back should you have to change/cancel your order for any reason.