LONDON (Reuters) – British Prime Minister Boris Johnson will pay tribute to Queen Elizabeth on Thursday ahead of four days of celebrations next week to honour her 70 years on the throne, describing her as the nation’s rock. Elizabeth, 96, marked seven decades on the throne in February, and nationwide events to commemorate her Platinum Jubilee, including a thanksgiving service and a music concert outside Buckingham Palace, will take place from June 2 to 5. In parliament on Thursday, politicians will deliver their tributes to the queen in a “humble address”, with the debate opened by Johnson. “T…