LONDON (Reuters) - Travel operator TUI has cancelled all foreign holidays for British tourists until July 1 due to the COVID-19 pandemic, sending shares in the German group down 6% on Friday. The company, which had already axed departures until the middle of June, said holidays to destinations such as Spain and Greece departing on or before June 30 had been cancelled, while its lakes and mountains trips had been stopped until October 1.It also halted sailings on its Marella Cruises line until..