'Capable' is the pivotal term in this context - MB opted to establish it as a permanent display, replacing the ambient temperature indicator; nevertheless, there is no obligation to maintain it as a permanent feature - this decision rests with MB.
I owned several vehicles (non-MB) equipped with digital screens that allow toggling between mph and km/h, so fulfilling the legal need. It is unnecessary to have it permanently shown.
Upon executing the 'UK Delete' modification (i.e., reverting the instrument cluster from UK specifications to non-UK), the vehicle remains compliant with UK regulations, as it retains the functionality to display speed in km/h, selectable via the instrument cluster menu.
Nonetheless, as previously mentioned, this action will inhibit the use of the LCD display in the instrument cluster for any other purposes, perhaps prompting MB to provide this UK option for convenience. However, the vehicle will remain compliant with UK regulations despite using the non-MB variant of the instrument cluster.