Section 3 Theft Act 1978
Which states:-
(1) Subject to subsection (3) below, a person who, knowing that payment on the spot for any goods supplied or service done is required or expected from him, dishonestly makes off without having paid as required or expected and with intent to avoid payment of the amount due shall be guilty of an offence.
(2) For purposes of this section 'payment on the spot' includes payment at the time of collecting goods on which work has been done, or in respect of which service has been provided.
(3) Subsection (1) above shall not apply where the supply of the goods or the doing of the service is contrary to law, or where the service done is such that payment is not legally enforceable.
A couple of issues to bring to your attention. As you can see in subsection 1 above, there are two points to prove
a) That the person knows that payment is required or expected
b) There has to be a dishonest intention to avoid payment.
Part a) is easy to prove, it is common knowledge that you pay for petrol at a petrol station - it isn't free.
Part b) is slightly more difficult to prove since it is a state of mind prior to taking (in this case) the petrol. It would be down to the circumstances such as driving in filling up, not going to the cashiers desk and driving off the forecourt or perhaps having false number plates on. Even the fact that someone doesn't have any money on them isn't proof per se, due to the fact you could genuinely leave the house without your wallet or purse. It has to be an investigation based on all of the facts before the CPS would authorise a charge of Making off without payment.
In the case above it is difficult since if the cashier says "anything else" and the customer says no and then the cashier simply tills in the sweets or drinks the customer is clutching, is it a deliberate act on behalf of the customer or a mistake by the cashier? It could genuinely be the correct answer by the customer - anything else? no (just the petrol and these sweets) - some people are genuinely frugal with words. This is why all Cashiers should ask the customer if they have had any petrol. If they say no at that point then a dishonest intention is easier to prove,
In a lot of cases the investigations are followed up and the customer returns to pay for the petrol. I'v lost count how many times I've been asked to go see the registered keeper and they genuinely haven't got a clue why I am there. It soon becomes apparent when it is a genuine mistake.
There are many cases where people haven't paid for the petrol at the time - the common one is the case above, then you have elderly or confused people and people suffering from mental health problems - there are probably a lot more but it isn't as simple as saying "If you don't pay for it you've committed a theft". This is why the law is framed in this manner, Parliament are quite wise in many cases and realised that there is a world of difference between a criminal intention and a confused mistake. The law isn't out to penalise genuine errors.
One last thing - the Petrol station do NOT have a right under this law to impound your car or any other item. Whilst it is the norm for say a mobile phone or watch to be left as a guarantee you will return, it is purely a civil agreement between the petrol station and the car driver.