(Reuters) – Britain’s Virgin Media O2 plans to launch a fibre network-building joint venture with owners Liberty Global and Telefonica SA, as the group aims to take on BT Group PLC in the high-speed broadband market, the Financial Times reported on Sunday. Telefónica and Liberty Global, who oversaw the merger of Virgin Media and O2 to create the enlarged group last year, will begin contacting potential investors this week but some informal discussions have already begun, the report said, citing a person familiar with the matter. The new company is seeking a substantial sum from third-party inv…