LONDON—British retail sales slumped in December after consumers did much of their Christmas shopping earlier than usual in November and many people stayed at home due to the spread of the Omicron COVID-19 variant. Economists said the scale of the hit bolstered their expectations that the world’s fifth-biggest economy shrank last month under the strain of Omicron and new government restrictions to slow its spread. Sales volumes fell by 3.7 percent from November, a far bigger hit than the 0.6 percent decline forecast in a Reuters poll of economists and the biggest fall since January of last year when the country was under a COVID-19 lockdown. Compared with December 2020, sales volumes were down by 0.9 percent, the Office for National Statistics (ONS) said on Friday. The Reuters poll had pointed to a 3.4 percent rise in sales in annual terms. “After strong pre-Christmas trading in November, retail sales fell …