(Reuters) - SSE has agreed to sell its British retail energy business to OVO Group for 500 million pounds, catapulting a relative newcomer to a leading role among the country's electricity suppliers.SSE's retail energy business is the third-largest supplier of the "big six" in the British market, supplying energy and related services to around 3.5 million household customers.Ovo, set up only 10 years ago, has expanded to be Britain's largest independent energy supplier, with 1.5 million..