I've never been to Plymouth, so I can't speak for it, but knowing other cities picking anything up off the street is always a bad idea!
Surveillance CCTV is so easy these days, not only from fixed cameras, but also mobile ones in parked up vehicles. You don't need to stop to be busted either, just driving slowly (having a perv), or passing a couple of time is enough justification for getting stopped.
It is not unknown for plain clothes police wimmin to get dressed up and 'walk the street' as well.
Second point to watch is the scams. These are very common, as you will have no way of finding the girl again. At least with an 'in call' you can go back to the house if you have a problem. And do you want to be dealing with a nasty prossie out of her head on crack in your nice motorcar?
And thirdly, you can pretty well guarantee any street girl is doing it because she needs cash desperately, (probably for drugs), rather than a genuine 'service provider' who is looking to build up a business from satisfied customers.
Please, have a good think first!!