LONDON, Sept 21 (Reuters) - Investors have placed more than 90 billion pounds ($123 billion)in orders for Britain’s first green government bond, one of the bookrunners on the transaction said on Tuesday -- a figure which would exceed all previous records for British government debt sales. Books opened on the 0.875% July 2033 gilt at 0800 GMT and close at 0900 GMT. Price guidance for the gilt has been tightened to 7.5 basis points over the benchmark June 2032 gilt ($1 = 0.7307 pounds) (Reporting by David Milliken, editing by Andy Bruce)