LONDON (Reuters) - Taking on the Big Four accounting firms will remain a lengthy and costly challenge for smaller rivals in Britain despite the regulatory leg-up unveiled on Thursday, but France has shown it can be done over a decade.The Competition and Markets Authority (CMA) has ordered that Britain's top 350 listed companies must hire two auditors to loosen the grip of EY, KPMG, Deloitte and PwC that individually check the books of all but nine of those companies.It forms a package of..