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The nude scene starts at around the 14:45 mark in the episodes. Her breasts are shown a number of times during a sex scene but it is in dim light and the cuts are very brief. The best views are after the guy gets off her and she goes to snort some coke. There is then a close-up shot of her lying flat on her back with her erect, very large, nipples sticking out in decent lighting..
From Recapped.com:
Lucy Walters, the subway girl from Shame, does her first nude scene in episode 4.