Antonia appears in episode 2 of season 2 of The Afternoon Play, "Sons, Daughters and Lovers," which aired in 2004.
In the episode she plays Kate Sullivan, an 18 year old girl living with her single mother Anna (Caroline Langrishe). She frequently argues with her mother who she sees as overly cautious and set in her ways. Kate meets and begins dating a much older man, Hardy Rose (Michael French). Simultaneously, her mother Anna meets Hardy's 18 year old son Ben (Liam Garrigan) and starts hooking up with him. Hijinks ensue.
Another role featuring very young Antonia playing a teenage girl. Kate's a bit of a brat, both as a flirt and when arguing with her mother, but that's appropriate for the character and story and Antonia plays it well. I don't generally go for the "girl fawning over a man old enough to be her father" trope but she has good chemistry with Michael French and despite some weak dialogue I enjoyed her flirtation with him. I particularly enjoyed her relationship with her mother - Antonia and Caroline Langrishe had great chemistry and it was interesting to watch Kate simultaneously feeling frustrated by her mother's lack of spontaneity while also being quite protective of her. Overall I found the story entertaining and I'm a bit sad I'll probably never see the end of it as it's lost footage at this point (more on that below).
The Afternoon Play isn't available on DVD or streaming anywhere that I've found. Thankfully someone was kind enough to upload most of the episode to YouTube. Unfortunately the quality is poor and the end of the episode is missing, but I'm grateful at least some of it was preserved.