British retail sales slid further this month, with sales far below the norm expected for this time of year, according to a survey on Thursday that added to signs of a slowdown underway in Britain’s economy. The Confederation of British Industry said its monthly retail sales balance, a gauge of sales volumes over the past year, fell to -54 in June from -46 in May. For the three-month average to June, the balance fell to -56 from -55, the lo…