The UK's ATC is split into the following categories; Delivery, Ground, Tower, Approach, and Centre (En-Route). These are also called positions. The idea behind these positions are to ensure that controllers do not deal with more aircraft than required, and each positions its own responsibilities.
Depending on traffic levels and airspace complexity, some airports may have more than one of each of the positions. In fact, it is common practice for busy airports, such as Manchester or Heathrow, to have more than one Tower, Approach and even Ground controllers.
Some airports might not even have one of these positions, such as Leeds which does not have a Ground controller, or Doncaster which does not have either Delivery nor Ground. In such a case, the position above will take-over the responsibilities for the underlying position. This is also called top-down controlling.