Ignorance of the girls age is not an excuse to break the law, and the people who procured snd trafficked her would have a hard time arguing they did not know her age.
In England it is illegal to have sex with a minor trafficked for prostitution.
And if you insist that she was a prostitute, then while English law does not penalize prostitutes it does stipulate that participants must be over the age of 18.
In this respect procurement of people Under the age of 18 for prostitution and participation in sex with a person under the age who has been procured for prostitution is offense under Clause 47 the UK’s Sexual Offenses Act 2003.
Moreover, a U.K. resident who has sex with a minor overseas, in this case NY, and where that minor has been trafficked for the purposes of prostitution commits the offense under Clause 47 the Sexual Offenses Act 2003 as stipulated under Clause72 of the Act.
https://www.legislation.gov.uk/ukpga/2003/42/part/1/crossheading/offences-outside-the-united-kingdom/enacted