While "There are" is proper English in North America "There is" in terms of measure is far more frequently used.
I'm an American, and "There are eight centimeters left" sounds much more natural to me than "There is eight centimeters left". I guess it wouldn't totally surprise me to hear the latter used, and I can sort of see an argument for it (the "eight centimeters" is being considered as a single quantity), but it sounds a bit off to me, and it's definitely not how I'd ever phrase it.
Maybe it's a regional thing that depends on what PART of North America you're in? (I live in California.)
Difficult to understand why "there ARE eight centimetres left" and "it IS two hundred kilometres from Dublin to Cork".
“There” expresses an existential quality. The eight centimeters still exist. They are still there.
“It is 200 km” is the answer to “what is the distance from D to C?” - “The distance/it is 200 km.”