By William Schomberg LONDON (Reuters) – Britain’s unemployment rate fell to its lowest since 1974 at 3.6% in the three months to July as more people left the labour market, according to official data on Tuesday that could add to the inflation headache for the Bank of England. Economists polled by Reuters had expected the jobless rate to stay at 3.8%. The Office for National Statistics said the number of people in employment grew by 40,000 in the May-July period, less than a third of the increase expected by the economists polled by Reuters. The economic inactivity rate – measuring the share of…