On UK-spec cars, MB replaced the ambient temperature display with the speed in km/h.
This way, the speed in km/h is always displayed, and the ambient temperature is only displayed when the 'Standard' display is selected. This was done to ensure that when driving on the Continent, the speed in km/h is always displayed.
On some models, it is possible to convert the UK instrument cluster display back to the European setup, this is known as 'delete UK mod'. An independent MB specialist with STAR should be able to do this for you if this option is supported for your vehicle.