LONDON (Reuters) -Worries about persistently high inflation in Britain grew on Wednesday as key measures of price growth monitored by the Bank of England failed to ease in July, despite a sharp drop in the headline inflation rate. The annual consumer price inflation rate cooled to 6.8% from June's 7.9%, the Office for National Statistics said - as the central bank and a Reuters poll of economists had predicted and moving further away from October's peak of 11.1%. But fresh signs of stickiness in core inflation and consumer service prices echoed warnings by BoE policymakers this mon...