If you’re talking about dealer deals/offers, it comes down to the dealer in question – I picked up a car today and the agreed £250 discount was taken off of my payment on collection. My previous car the dealer threw in a couple of upgrades and a set of winter car mats worth about £160 without charge. Others may do a genuine “Cash-back” where they send you money by cheque/bank transfer after the deal is complete. Motability doesn’t officially support or endorse any discounts or cashback systems that I know of, so there is no standard across the brands for how the deals work.
"Man is born in freedom, but he soon becomes a slave, in cages of convention, from the cradle, to the grave."