My advice would be to ask Motability to do the work for you. Ask them to call the dealership and confirm it has a build slot, when they lie to motability things are strangely taken more seriously. Less stress, easy trail for cancellation and quite satisfying knowing the sales ‘executive’ can’t say anymore to keep the order.
Perhaps it’s just me, but I like to see the whites of their eyes and ask myself! My fear would be that 90% plus of the staff of the Motability help desk probably don’t understand the complexities of build slot allocations either, so I wouldn’t trust them to ask the right questions. They will just ask “have you ordered the car?” and the dealer will say “yes”! Perhaps it’s just a cynicism that comes with age, or several bad experiences, including my first Motability order. Either way, car dealers that tell it straight are a rare breed and I find that when they realise that you know what you are talking about and ask the right questions the BS tends to subside a little!