2/3 of Britons cimmit petty crime....are they all really crimes ?

Very often the person giving you too much change isn't dealing with his or her own money, but with that of the employer. By deliberately accepting money to which you know you are not entitled in this way, you are committing an offence of dishonesty -- provided, that is, that you notice the mistake. There are obviously occasions in our lives when we're hurrying off and don't bother to count the change properly and if it is not a significant amount, might not notice the discrepancy. In that case, we would lack the mens rea -- the criminal intention -- and so would not be committing a crime. But once you know that this money belongs to another and the person handing it has no authority to do so, then you are obtaining pecuniary advantage by deception, the deception being to keep your mouth shut when you knew that the honest thing to do was to point out the mistake.