I am not surprised that no action is being taken but not for the reasons the OP suggests. There simply isn't the evidence.
If she had sex with Andrew (which he denies), she was over 16. The fact that she may have been under the age of consent in some states of the USA is irrelevant. She was in this country, so UK laws apply. Her legal action the US is partly based on her being under 18 at the time of the alleged sexual encounters, but she appears to have been over the age of consent in the locations where the alleged encounters took place.
Even if she was trafficked and Andrew did have sex with her, no offence was committed. Whilst it is now an offence to pay for sex with someone who has been trafficked even if you are not aware that they have been trafficked, it was not an offence at the time and criminal laws do not apply retrospectively. Also, she has not alleged that Andrew paid for sex so, even if the encounters had happened after the law changed, Andrew would not have committed any offence.
She alleges that she was raped but the allegations in her legal action are sufficiently non-specific to mean that they fall some way short of proving beyond reasonable doubt that she did not consent and Andrew knew she did not consent.
Cressida Dick was at school with Ghislaine Maxwell
Social media has got this wrong. They were not at Oxford High School together. They both attended this school but at different times. Despite its name, the school caters for ages 4 upwards. Maxwell started her education there and left in 1970 or 1971 whereas Dick did not join the school until 1974.