In English law, theft is a criminal offence, which covers almost all forms of taking. It is a statutory offence contrary to Section 1 of the Theft Act 1968. English law is the law of England and Wales, rather than Scotland and Northern Ireland. ... The Theft Act 1968 (1968 c. ...

The 1968 act was an attempt to rationalise a nebulus and unnecessarily complex set of precidents and doctorines in common law.

It was only partially successful and needed at least the further Theft Act 1978 to strengthen it. The Theft Act 1978 supplemented to earlier Theft Act 1968. ...

Elements of theft

Theft consists of the following elements: (1) there must be property, which (2) belongs to another; and (3) is appropriated. The appropriation must be (4) dishonest; and (5) done with the intention of permanently depriving the other of it. Items (1)-(3) would normally be classed as forming part of the actus reus and (4)-(5) of the mens rea. Actus reus is the action (or inaction, in the case of criminal negligence and similar crimes which are sometimes called acts of omission) which, in combination with the mens rea (guilty mind), produces criminal liability in common law based criminal law jurisdictions such as the United States, United Kingdom. ... Mens rea is a criminal law concept which focuses on the mental state of the accused and requires proof of a positive state of mind such as intent, recklessness, or willful blindness. ...

The most surprising aspect of this definition to the non-lawyer is that there needs to be an intention to permanently deprive someone of the property. This means that, in general terms, "borrowing" is not stealing.

However, in regards to appropriation, if you assume an owner's right over the object, it can become theft. For instance, if you were to relabel the price of an item in the supermarket, you would have assumed the right of the owner (appropriation) over an item (property) that does not belong to you. By relabling it you have been dishonest and finally, by changing the value, you have permanently deprived the other of some of its value. You would therefore have committed theft.