Im worried that foster care will make her even worse in terms of the long term outcome, most girls who go into care dont do well.
Well, she isn't doing well now, is she? I don't see how she could do much worse than she currently is. She hasn't been to school for three years, she's a criminal, she's violent and abusive and a risk to you and your unborn child. You've said she's "banned from most relatives' homes" so clearly, her issues are not just with you but with other people too. Her long term outcome is already bad.
I would never, ever say this lightly ... but I do think you are looking a situation where you might have to consider asking SS to put her in care for everyone's sake. If you did, then she would at least be on SS's radar and have more attention from them than she currently has.
She may have some mental health issues, but I think the counsellor she 'took the piss out of' would have been able to make some guess at what those issues might be even if she 'refused to engage'. A counsellor can still observe a child's behaviour and refusal to engage says something in itself. It's also worth remembering that you can have mental health issues and, quite separately, also be a nasty piece of work. Mental health problems are not a blanket explanation for the behaviour you're describing.
From what you've said, I do think you're being way too soft on her. For example, why are you letting her boyfriend in the house all the time if you don't like him being there? Why don't you tell him he's got to leave? Your daughter may well tell you to fuck off, but I doubt he will - and if he does, then that's it, you call the police and say he is refusing to leave.
I wouldn't be giving her any allowance at all, not even the £20. I would also be giving local shopkeepers a picture of her and telling them she is underage and that if they continue to sell her fags you will report them for it. Why does she have the latest clothes and gadgets, ffs?
How long have you been with you partner? I'm guessing 3ish years?
The OP did say her daughter was a difficult and destructive child even as a toddler and all the way through primary school. Her problems in general didn't start three years ago, only the school refusal.