If they can’t fulfil the order within the year, give the customer the option to wait. Motability absolutely accustoms waiting times, even at that length.
The only reason not to do that is twofold: either they don’t want to sell the car to a Motability customer, or B: they didn’t order the car immediately after the customer left the premise and sat on the order until the car was removed from the scheme, at which point Motability wouldn’t accept the order anymore.
Don’t make the mistake and think the dealership “lost out” or “is sorry”. They’re not. They’re solely responsible for this.