No the closer equivalent to R-18 would "unrated" since it is used for porn (but not always). Now I saw someone say that US and UK ratings look something like this
U=Y, G, Y7
PG=PG
12=PG-13, 14
15=PG-13, 14, R, MA
18=MA, R, NC-17
R-18=literally just porn
as for PEGI and ESRB
3=E
7=E, E10+...