I expect they have been reported. There seem to be drug dealers everywhere. A couple we know reported to the police that people were dealing in an alleyway near their house. Nothing was done. We have a rather nice little park near us - a playing field where people walk their dogs and a children's playpark. There are often solitary men sitting on the seats, looking constantly at their mobile phones, obviously either waiting for a delivery or waiting for a customer.
I think the issue probably is that they are seen as "small fry" and the only reason the police might be interested in their movements is to see who, further up the line, is supplying the drugs for them to sell, and so on up the chain. In order to arrest and charge people, the police need to follow exactly the right procedures and obtain proof of a drug transaction involving money taking place, which can be quite tricky.
I'm inclined to agree with Professor David Nutt who the Labour government got rid of. He said the whole drug culture should be de-criminalised and drugs sold, in a controlled way, out of pharmacies. At least it would cut out criminal involvement and the grooming of children to transport drugs around.