Market leader Tesco's TSCO.L sales were up 6.5%, with Sainsbury's SBRY.L up 6.6% and Asda up 7.7%. By James Davey LONDON, Dec 13 (Reuters) - British shoppers focused on core grocery essentials in November and avoided more discretionary Christmas items, market researcher NielsenIQ said on Tuesday. "With double digit food inflation, shoppers will be spending more to buy less this Christmas," Mike Watkins, NielsenIQ’s UK head of retailer and business insight said.