The 18-rating was used exclusively at the BBFC under the notorious serious James Fermin during the 1975-99 period. Nowadays, they are generally portrayed in films depicting coercive and violent acts (especially suicide), sexual content (and especially sexual violence), use of racial language or illegal drug use.
Now, you might be thinking that these standards still apply to a lot of movies that you've seen, and you're right - crossing is a fine line for getting 18 in general. Taking something like John Wick Chapter 2, which is extremely violent - the only kit needed to get the 15 ratings was to remove the "bloody injury detail in the suicide scene".
It is debatable that this reduction in 18 certificates can be explained by a general change in which society is deemed acceptable.