LONDON—Tens of thousands of people in Scotland and northern England remained without power Sunday after a storm brought sleet, subzero temperatures and disruptions across much of the United Kingdom. Icy gusts caused power cuts around the border between Scotland and England, with Northern Powergrid saying it recorded 1,100 instances of damage requiring significant repairs. It said it was trying to restore service to 40,000 customers, while the SP Energy Networks said 21,000 customers remained without power Sunday. “The storm was well forecasted and despite being prepared, Storm Arwen resulted in damage of a scale and intensity not seen for 15 years,” said Rod Gardner, Northern Powergrid’s major incident manager. The power cuts came as forecasters issued ice warnings and said the coldest night of the season would hit parts of the U.K. later Sunday, with snowfall expected across the country and temperatures dropping to as low as -10 degrees Celsius …