Because of there old BT Monopoly they are still the largest MVNO operator in the UK, you know those little brands that popup with great deals mostly SIM only? Nearly all of them use 02 infrastructure and are partly owned (at a minimum) by them as well and often get folded in to them again once they have market capture.

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Same goes for Broadband as well, most of the company's are just resellers of TalkTalk or BT or in some places Cable and Wireless (Tesco BB was a mix of the old Buldog LLU network and Virgin Media, they had to run two seprate call centers for them both outsourced) under two differnt brands as well - This was back in 2009 / 2010 but I do know from a friend who works for a network provider that Tesco have a LLU service from them but I am not sure if that's just the stores or something they are reselling.