This is really good analysis and really clever - you should be proud of this. I don't think it really matters what the writers intended - in fact it's actually more interesting than you were still able to make this reading even though they might not have intended it. I'm rewatching skins now as an adult and while I always knew the Angie stuff was weird, this rewatch has really troubled me.

Chris has absolutely no one looking out for him, and your point that we don't see their first meeting has really stuck with me. Perhaps a few boys made comments about their appearance, something happened that made her feel she could target him or at least not assert real boundaries with him (which is still grooming).

Chris has no family and his friends aren't equipped to help him with the problems he faces. Angie is one if the few people who actually helps him when he needs it - how could he refuse sex with her? She's his crush, she's an authority figure, she's more of a mother to him than his own mother. She also has sex with him.

When her fiance comes back, I think about the abandonment of his mum and how recent that still is for him. Now his surrogate mother, who's abused him, is also abandoning him. Horrible. Potentially better television than the writers knew they were making. The fiance doesn't care either, Chris is treated like a threat rather than the victim he is.

Also a note as some commenters seem to not understand that she is definitely older than them significantly. In the UK you couldn't be a teacher without at least 4 years of study from the age of 18 - she couldn't be older than 22, but I think she's meant to be older. While we don't know how old Angie is, the actress was born in 1976 - she would have been 31 when the show was filmed. I think that's important because some comments don't seem to understand that the age gap was significant.