It very much depends what vehicle your after and what stock is available.
You also have to remember that dealers pay for their vehicles they may have in stock and aren’t obliged to let you have it. Equally Motability buy the vehicles directly from the manufacturer and not the dealership, who are just a middle man to allow us to place orders with and to order from the manufacturer, to which they receive a payment from Motability for ding this!
However if a dealer has over stock or stock they wish to shift, the dealer will at their discretion allow it to go to a Motability customer.