Do you mean the digital speedometer display, or the mileometer reading?
AFAIK If it's the digital speedometer, you can't change it as both MPH and KPH are required by UK law of being capable of being displayed.
That said, I understand that there are a couple of independent dealers who can change it to MPH via Mercedes STAR diagnostic and programming machine, using UK delete.
If it's the mileometer and it's the same as later models it can be changed by accessing the OBC using the steering wheel buttons. (Assuming the MY 1999 has them).
Have you used the MB Club search option, as there's probably several threads on the subject.