By Andy Bruce and William Schomberg LONDON (Reuters) -British consumers are feeling the strain of a tightening cost-of-living squeeze with retail sales sliding fast and consumer confidence approaching all-time lows, according to data that sent a fresh warning sign for the economy. Retail sales volumes slid by 1.4% in March from February, a worse reading than any economist forecast in a Reuters poll that pointed to a 0.3% monthly drop, the Office for National Statistics said. Earlier on Friday, market research firm GfK said consumer confidence fell this month close to its lowest level since rec…