LONDON (Reuters) – Britain’s Jake Dennis won the first round of a London Formula E double-header on Saturday while Stoffel Vandoorne finished second for Mercedes to increase his lead in the electric championship. Dennis led from pole for the Avalanche Andretti team to secure his first win of the season. It was his second triumph in London after also winning last year at the ExCel centre. Dutch driver Nyck de Vries, the Formula E champion, was third for Mercedes. Belgian Vandoorne now has 173 points in the drivers’ standings, 26 clear of Jaguar’s New Zealander Mitch Evans who finished sixth on …