Actus reus - Appropriation
Section 3 of the Theft Act 1968 - 'any assumption by a person of the rights of the owner'.
Appropriation means that the defendant has taken something. Obvious examples of this is where the defendant is physically taking something e.g. pickpocketing. However, theft can also be where someone destroys property, sells property, switching price tags etc.
Regardless of the way they have stolen, they are essentially assuming the rights of an owner when they take something that does not belong to them.