Either "kph" or "kmph" is a casual, sloppy, but common way, along with abbreviations like "gms" (grams) or "mts" (metres). None of these are unusual in the UK.
The correct way is km/h, or (occasionally, for mathematicians or scientists) km·h⁻¹ or km h⁻¹.
The correct way is km/h, or (occasionally, for mathematicians or scientists) km·h⁻¹ or km h⁻¹.