I suspect the non-trafficked ones do the job to achieve a particular goal e.g. make enough money to do x (help their family/children, pay bills, get surgery, buy house, pay hospital or medical bills, pay their way in college/university, etc.) and once they achieve their goal they return to their normal life. They may also have a bit of a vacation while they're in the country. Some may miss the money and return to the job over the years. Others happily achieve their goals and return home and enjoy their lives. The amount of time they spend in the country may be dictated by the length of their visa waiver.
I think the South American girls tend to be independents who gave travelled to the UK to have a secret job away from their family. They want to make money and see the country. They may work ir visit the country in groups possibly actually friends. At the end of their European tour they return home ehen their tourist visa/visa waiver expires.
The trafficked ones probably get worn out and disguarded and replaced by "fresh meat" in the grinder.