I've been a criminal law solicitor for more than 30 years with more than a professional interest in the laws of E & W relating to prostitution and that's definitely not true.
The police will have their own standards of evidence before it will get passed the custody officer. The offence is soliciting in a street or public place and the police need evidence of soliciting. Being in a car with a prostitute does not mean that soliciting has taken place. The punter could have picked her up in a brothel. The prostitute may be a friend or relative.
If I'm sitting in my car and a prostitute taps on the window and offers me a blow job for £20 and I take £20 out of my wallet and give it to her in return for a blow job then I haven't solicited her. She's the one whose solicited, not me.
A few years ago a guy I know in Bradford received a letter in the post from West Yorkshire Police asking him to contact them. He went to the police station spoke with a PC who said "we want to speak to you about why your car was seen in the red light district on cctv"
He explained "I work as a cleaner for the council. I clean schools and offices. To get from home to one school I drive along the road you have me on cctv. I clean there twice a week. He heard no more from them. In an episode of traffic cops also filmed in Bradford police approached a stationary car in a back lane. There was a man and known prostitute in the car just sitting. There didn't seem to be a reason for the police to speak to the occupants.
They requested id from both were unhappy with information supplied by the male and used a fingerprint scanner on him.
He was a disqualified driver in his brother's car hiving his brother's details.
At the end of the program they do a voice over explaining what happened to the people stopped.
In this guys case he was given a further 12 month ban 6 points for no insurance fined £ 250 for perverting the course of justice and £100 for what they described as 'kerb crawling'
I think it's initially down to the officer who stops you then the custody sgt and finally the CPS. From what you say this guy should not have been charged with either soliciting or kerb crawling as he was sitting in the car with the woman