LONDON, June 1 (Reuters) - British house prices rose by 10.9% in May compared with the same month last year, the biggest annual increase in nearly seven years, mortgage lender Nationwide showed on Tuesday. House prices were 1.8% higher than in April, Nationwide said. Economists polled by Reuters had expected prices to rise by 9.2% in annual terms and by 0.8% from April. (Writing by William Schomberg; Editing by Alistair Smout) Our Standards: The Thomson Reuters Trust Principles.