By Andy Bruce LONDON (Reuters) – British government bond prices tumbled on Monday in a sign that investors are yet to be convinced by Finance Minister Kwasi Kwarteng’s drive to shore up fiscal credibility, which included bringing forward his new fiscal plan to Oct. 31. The plan had previously only been due out on Nov. 23 – two months after Kwarteng triggered a rout in British bonds by announcing a “mini-budget” that included 45 billion pounds ($50 billion) of tax cuts but no details on how they would be funded. The finance ministry also announced that a civil servant with extensive budgetary e…