By Sinead Cruise and Iain Withers LONDON (Reuters) -HSBC is reviewing whether to keep its global headquarters in London’s Canary Wharf, a staff memo seen by Reuters on Thursday showed, a decision which could see the bank swap the financial hub for a new home after more than 20 years. The bank said it wanted to assess its “best future location in London” ahead of its lease expiring at the 45-floor tower at 8 Canada Square in early 2027. HSBC, which is currently defending itself against calls for a break-up from its top investor, has long grappled with the optimal location for its business, with…