The companies looking after the electricity infrastructure are called Distribution Network Operators (DNO), and the supply companies, who use the network to sell and distribute electricity, are called energy suppliers. Many of these supply gas as well as electricity.
There are 14 DNOs operating in the UK, although some of these are owned by the same parent company. Their geographical boundaries more or less match the regional boundaries of the old ex-public electricity companies. The main DNOs in the South East are EDF Energy Networks and Southern Electric Networks.
There are also Independent Distribution Network Operators (IDNOs) who operate small electricity distribution networks. These are often mains extension projects that they previously carried out for large new housing developments.
Currently the UK has 20 electricity energy suppliers and these companies are not limited by geographical areas, so there is a wide choice for consumers. These include British Gas and EDF Energy. The latter owns the EDF Energy Network company above but operates separately according to regulatory requirements.