"Just going for change" is a phrase that in any appeal to a parking company will raise a smile and a mental tick that here is someone that will be paying a parking charge - one way or another. It is by common understanding that anyone visiting a Pay and Display car park will do so knowing they will pay before leaving the car park. Regrettably nearly 57 years after the key case of Strong v Dawtry  1 All ER 926 people still believe they can leave then pay. This also applies to sites which have Ringo or similar online payment processes.
Some car parks will allow 15 minute "grace" period which can allow a degree of leeway. A couple of car parks specifically allow up to 24 hours to pay late - but only if the terms on the signs are specific about delayed payment.
Synopsis: Appellant was the driver of a motor car which he parked at 11:5 am in a parking bay. Finding that he had no change with which to pay the initial charge into the parking meter he approached a police constable standing approximately 15 yards away and asked him for change but he was unable to give it to him. Appellant obtained change from a taxi-driver and returned at once to make the initial payment. While he was obtaining change a parking meter attendant arrived at the parking meter which showed an excess of thirty minutes and attached to the car an excess charge notice indicating that at 11:10 a.m. an excess charge of 10s had been incurred. While he was attaching the notice appellant approached and told him that he had left the bay for two minutes immediately after parking his car there for the purpose of obtaining change. He then paid the initial charge into the meter. Appellant was convicted of unlawfully failing to pay the initial charge prescribed by the Parking Places (Westminster) (no 1) Order 1959 article 8 (1), contrary to Road Traffic Act 1956 (c 67) s 22 (1) (a) (repealed).
On appeal: Held appellant was rightly convicted because the words ‘payable on the leaving of the vehicle in the parking place’ in article 8 (1) denoted that payment should be made as soon as the vehicle was placed in the parking place; and no qualification for allowing a reasonable time, according to circumstances (including the obtaining of change) between getting out of the car and inserting the coin into the meter, could be imported into the words of article 8 (1).