(Reuters) - Britain's competition watchdog has flagged concerns about potential price increases at some pubs from a merger of Slug and Lettuce pub owner Stonegate and rival Ei Group, which would create the country's biggest pub operator.The Competition and Markets Authority (CMA) said on Friday Stonegate Pub Company's proposed purchase of Ei Group did not raise UK-wide competition concerns but could damage competition in 51 local areas."The CMA is therefore concerned that, if the businesses were..