Are you okay, mate?
Report her to the police. Just say you paid for a massage. Nothing embarrassing about that. Police generally don't give a toss but there's a chance that they're wanted for something else and it might tip the balance to get them banged up or something.
Do you have a general description of the scum bag pimp? Age, ethnicity, clothing, hair colour etc?
Perhaps some proactive punters in the Derby area could do a few dummy bookings and take a picture of the car and people involved? From a safe hiding place, for example.