OP, the way you present the circumstances with your DD is very shallow and odd.
it's all about £20 for this and £50 for that - but what has actually happened???
When did the relationship break down?
Was there a time you got on better?
Was she like this when she was 14? 12? 10?
Is there something else going in in her life she is struggling with?
Even the fact that you can come on an anonymous forum and ask, "Should I cut my DD off?" and then list the amounts of money she has asked you for and what you give her and what you don't give her..., this says it all!
Even when you tell people about your son, the piece of info you choose to share is that he doesn't ask for money!
No wonder she is frustrated. She is asking for money because this is the only thing you seem able to understand.
She is 18 fgs! She is in sixth form! She is being fed and housed by another family and you give them nothing! Aren't you ashamed about that? What does your husband say? Where is he in all this?
This relationship with the bf will break down eventually. Then what? You are storing up much bigger problems for the future because if she goes off the rails now, it's going to be hard to get her life back on track in her 20s.
Do you ever try actually talking to her about what's going in in her life? Not reacting, but just listening? Do you try and understand, rather than just focusing on the here and now and money?
Let her come and do her laundry. Let her come for a takeaway. It's better than nothing. Calm down. Make it clear the door is open and that "cutting her off" is not an option. Stay calm. She is testing you. She is testing you because she knows, deep down, that you are not unconditionally there for her. Something is deeply broken. If a young person has the type of parent who genuinely would consider "cutting them off" - this is a very painful knowledge to come to terms with and generates the type of insecure / aggressive behaviour you are currently experiencing. She is pushing you to cut her off because she thinks you actually might do it.
I'm sorry if this sounds harsh, but you and your husband are the adults here. Yes, she is 18, but so what? She is a child - in full time school. She needs to know that you are there unconditionally for her. The money is not the issue at all.