As with quite a few industries in the UK, the supply of natural gas to homes and businesses used to be a nationalised industry. And, like a few other industries, this business was privatised by the Conservative
government in the 1980's.
British Gas was spun off as the sole provider and transporter of gas, but in 1993, the Monopolies and Mergers commission decided that there should be some competition. The company was split into two, Centrica (which sold the gas) and BG plc, which was in charge of making sure it got from refinery to property without leaking anywhere.
Further splits, buyovers and takeovers meant that the Transco piping division of BG ended up in the hands of Lattice Group plc, and people selling gas ranged from people you might expect (Centrica, still trading as British Gas), to electricity companies and, of course, Virgin. (they're everywhere!)
Transco is still around, though - they maintain the pipe network, check for leaks, and do essential repairs. If you smell gas, Transco are the people to call.
Just about the only time the public need to get in touch with Transco are if they need a property hooked up or disconnected to the gas network, or if there's a problem with the supply. They don't need to pay the company directly. Instead, Transco are funded by the gas suppliers.