LONDON (Reuters) - Tesco , Britain's biggest retailer, ground out a 0.1% rise in underlying sales in its home market during what it said was a "subdued" Christmas for consumer spending.Showing how hard supermarkets had to work over the festive period, Tesco said it had cut prices, delivered the best operational performance in six years and seen the biggest ever day of UK food sales in its history, just to secure a small rise in organic growth.British shoppers turned particularly cautious in 2019..