LONDON (Reuters) – Britain faces its largest rail strike in decades on Saturday as tens of thousands of workers across three trade unions walk out in disputes over pay and job security, shutting down most of the nation’s railway network. The strikes are the latest in a year marked by widespread labour unrest in Britain, involving everyone from transport and postal workers to lawyers, as the rising cost of living and rocketing energy bills outpace pay rises. With inflation hovering around 40-year highs of nearly 10%, labour unions say pay offers for their workers amount to a pay cut in real ter…