LONDON—Britain reported a surge in cases of the Omicron coronavirus variant on Saturday, which government advisers said could be just the tip of the iceberg, and London’s mayor declared a “major incident” to help the city’s hospitals cope. The number of Omicron cases recorded across the country hit almost 25,000 as of 1800 GMT on Friday, up by more than 10,000 cases from 24 hours earlier, the UK Health Security Agency (UKHSA) said. Seven people believed to have had the Omicron variant had died as of Thursday, up from one death in the UKHSA’s previous data, which ran up to Tuesday. Admissions to the hospital of people thought to have the variant increased to 85 from 65. The government’s Scientific Advisory Group for Emergencies (SAGE) said it was “almost certain” that hundreds of thousands of people were being infected with the variant every day and were not being picked up …