I mean generally there is nothing wrong with changing your mind, as many services are under discretion. They just might not like the look of you.
It is just when it is accompanied by other factors that should really drive home the fact that this isn't going to be a good time. In that scenario, it is always better to walk, you leave with your money, and the girl doesn't have to do something they don't want to as some people might insist a bit strongly.
We are lucky that we live in London, so there are always so many other options.
I can't speak for the rest of the country then, perhaps a wank at a local service station car park on the way back home?