LONDON—In a surprising result, British Prime Minister Boris Johnson’s Conservative Party has been easily defeated in a special election for a seat that it has held for decades. The Liberal Democrats, which was in a coalition government with the Conservatives between 2010 and 2015 before seeing its electoral fortunes wane dramatically, won Thursday’s election in Chesham and Amersham, 57 kilometers (35 miles) northwest of London. Sarah Green, the Liberal Democrat candidate, picked up around 57 percent of the vote and won a seat the Conservatives have held since it was created in 1974. She added around 30 percentage points to the party’s result from the 2019 general election. “This Conservative Party has taken people across the country for granted for far too long,” Green said Friday. Her party leader, Ed Davey, said the result sent a “shockwave through British politics” by showing that the “Blue Wall” of Conservative seats in …