GRANTHAM, England (Reuters) -Former British finance minister Rishi Sunak on Saturday described himself as the underdog in the contest to become Britain’s next prime minister. Sunak’s resignation helped trigger a revolt that saw Prime Minister Boris Johnson agree to step down after a series of scandals. Members of the ruling Conservative Party will vote for a successor over the summer, with an announcement due on Sept. 5. Sunak led all rounds of the voting among party lawmakers to reduce the field to two candidates. But it is foreign secretary Liz Truss who seems to have gained the advantage so…