Over the next few weeks, Sunil Paul and thousands of voters like him in this commuter town on the edge of London will be among the most important people in British politics: they could decide Prime Minister Boris Johnson’s fate. Johnson, who called a snap election to break the Brexit deadlock, is the local member of parliament and is battling to retain his seat. If Johnson wins the election, but loses in Uxbridge, about 20 miles from central London, he will have to stand down.