LONDON—Marks & Spencer was Britain’s fastest growing food retailer in the key Christmas quarter, market researcher NielsenIQ said on Tuesday, providing further evidence that the company’s latest turnaround program is gaining traction. NielsenIQ said M&S’s grocery sales soared 9.4 percent in the 12 weeks to Jan. 1 from the same period a year earlier, with growth accelerating from its previous report. In November, M&S said it had beaten forecasts for first-half profit and upgraded its earnings outlook for the second time in three months, sending its stock soaring on bets that one of Britain’s most elusive turnarounds could finally materialize. M&S shares were up 1.6 percent at 0915 GMT, extending gains over the last year to 92 percent. Some analysts believe M&S, which also sells clothing and homeware, could upgrade its 2021–22 profit outlook again on Thursday when it publishes its own third-quarter trading update. NielsenIQ’s data showed market leader …