By James Davey LONDON (Reuters) -Cash-strapped Britons are buying more cheap frozen food to help cut waste and cope with “unprecedented” soaring living costs, the boss of supermarket group Sainsbury’s told Reuters. Chief Executive Simon Roberts said shoppers were “watching every penny and every pound”, visiting stores more often but buying less on each trip, and using technology to monitor their spending to avoid “till shock” at the check-out. “In many ways there is no playbook for what we’re dealing with at the moment, these are unprecedented circumstances,” said Roberts, a 30-year veteran of…