UK: Boris Johnson hosting the Uk Governments daily coronavirus press conference, Part Eleven.

Professor Martin Landry discusses the support from the Oxford team and NHS in conducting a large-scale clinical trial across the UK. Over 11,500 patients were enrolled in three months, making it the largest trial globally. The trial tested six drugs, with findings showing hydroxychloroquine ineffective in hospitalized patients and new insights on dexamethasone.

What makes the clinical trial mentioned in the text the biggest in the world?

What is the significance of dexamethasone in the context of the trial?