Aug 31 (Reuters) - Britain's FTSE 100 index is seen opening down 13 points, or 0.19 percent, on Wednesday, according to financial bookmakers. * The UK blue chip index closed 0.25 percent lower on Tuesday at 6,820.79 points, as a drop in major mining and housebuilding stocks weighed on the market. * ARM: Shareholders in British technology company ARM approved its sale to Japan's SoftBank on Tuesday, marking the end of independence for the chip designer that powered the smartphone revo