LONDON, Aug 20 (Reuters) - British retail sales unexpectedly fell sharply last month, setting back their post-lockdown recovery, according to official data. Retail sales volumes dropped by 2.5% in July from June, the Office for National Statistics said. A Reuters poll of economists had pointed to a 0.4% month-on-month increase in sales in July. (Reporting by William Schomberg; Editing by Kate Holton) Our Standards: The Thomson Reuters Trust Principles.