Because of security concerns, left luggage lockers are largely a thing of the past. As Rocky suggests, you could visit a museum which has cloakroom facilities and leave your bag there until later. Otherwise you'll have to pay £9 to leave a bag at Liverpool Street Station or Charing Cross Station. (There are also facilities at other mainline termini). The high fees charged at railway stations are, in part, because every bag has to be X-rayed.