What iD shows are just commonly used values. Many of those values can be wrong, such as "tbd" and "90".
Ultimately, you can use any value you want, even "Iras_favorite_lamppost", but no data consumer will know what it is and know how to handle it so it'll be just ignored. So what's the point of putting all this effort if your effort gets thrown out.
"Lamppost" is a plain American English terminology. But as you've seen, OSM uses British English and also certain industry-specific terminology. So words we use in everyday life might be be the same as what's used in OSM.
Lamppost is just a generic word. Is it straight? is it bent? Is it split? Is it double? That's why we have those predefined values that describe different types. 