That's typically inaccurate Daily Mail reporting, Ladybirder!
The IMF report expects the UK's economic growth to be the highest within the G7 group of nations. The Daily Mail correctly reports that as "Puts Britain ahead of the US, Germany, France, Canada, Japan and Italy" (since those are the other 6 members of G7) but then goes on to inaccurately state: "Britain will be the best performing major economy in the developed world", conveniently ignoring countries with far faster growing economies, such as China, India, Brazil and many others.