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Telefónica O2 U.K. Limited (previously Cellnet Limited and BT Cellnet Limited and branded as O2) is the second largest provider of broadband internet access and telecommunications in the United Kingdom, behind Everything Everywhere.Its national headquarters are located in Berkshire and it is a privately owned subsidiary of Telefónica.Between 1985 and 1989, John Carrington was the CEO of British Telecom's Mobile Division. It was during this period that Carrington founded Cellnet, and launched the companies first cellular service in January 1985. was established as a subsidiary of Telecom Securicor Cellular Radio LLC, a 60:40 joint venture between British Telecommunications and Securicor respectively.
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After months of rumours and speculation, Peter Bonfield publicly announced on 27 July 1999 that BT had agreed to buy Securicor's 40 percent share of Cellnet for £3.15 billion. The company was later rebranded as BT Cellnet, and it became a part of BT Wireless, a group of companies owned by BT. Cellnet had 5 million customers at the time of its acquisition.
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BT Cellnet was rebranded as "O2" on 1 May 2002, along with other former BT subsidiaries; Esat Digifone in Ireland, Viag Interkom in Germany and Telfort Mobiel in the Netherlands. The rebranding was supported with a European advertising campaign, which began on 16 April 2002, across all four countries, at a cost of £130 million. The main launch campaign ran from 1 May and was developed by Vallance Carruthers Coleman Priest, working alongside brand consultancy Lambie-Nairn, whom created the "O2" brand identity.