It is possible to approach the expression "brutal ugliness" as one that foreshadows the ending.

In the story, Barry contrasts the "brutal ugliness" of the girl with the attractiveness of the man she is trying to proposition. Both the girl and the man are in the Red Lion pub. Barry is observing the two, and he notes that the man's features are uncannily similar to those of the previous six murder victims:

Barry was fascinated, compelled to stare, by the resemblance to Pat Leston, by more than that, by seeing in this face what seemed a quintessence, a gathering together and a concentrating here of every quality variously apparent in each of the six. And what gave it a particular piquancy was to see it side by side with such brutal ugliness.

The "brutal ugliness" of the girl is incongruent with her manner; she appears to be hesitant and shy in the way she flirts with the young man. Yet, there is little in her features to suggest refinement or beauty. Barry is disgusted that someone of her appearance would dare to make "overtures" to an attractive young man. However, Barry is fascinated by the exchange, and he is drawn to the girl.

The young man the girl was propositioning eventually leaves, his "harsh laugh" lingering in the wake of the girl's pleading offer to buy him a drink. At this point, we are reminded of the descriptive expression "brutal ugliness"; it injects an obvious ominous tone to the story. Indeed, Rendell is not yet finished with the girl. In fact, she becomes a focal point of interest in the last two pages of the story (the other being Barry himself). 

We are further led to surmise that the expression "brutal ugliness" foreshadows an approaching danger after the author makes four important revelations: first, Barry never wears a scarf; second, all six of the murder victims were strangled with their own scarves; third, the girl pulls off her scarf while she walks with Barry; fourth, the girl hates men. 

By the time the girl cruelly points out Barry's ugliness and small stature, we are sure of impending conflict. Rendell does not disappoint us: the two engage in a physical struggle to the death. The ending is incredible: not only does the girl kill Barry, she is also revealed to be the murderess responsible for the previous deaths of six individuals (all of them male). Thus, the expression "brutal ugliness" foreshadows the ending perfectly.