Dec 18 (Reuters) - Britain's FTSE 100 index is seen opening up 8 points on Monday, according to financial bookmakers. * BHP: ExxonMobil Corp and BHP Billiton Ltd have agreed to end a nearly 50-year-old gas marketing joint venture in Australia, bowing to pressure from the nation's competition watchdog amid concerns about gas supply and soaring prices. * Sky: A hedge fund, which has a stake lower than 1 percent in Sky Plc , re