I'm not sure about England but in America its mostly rescues that have adoption days in pet stores because they are a network of foster homes and do not have a facility for people to come and see the animals. That is basically the benefits for the rescue, getting the animals out that normally may not be seen. It can work with shelters where people tend to pass over the older animals. The petstore may benefit if people buy products for their newly adopted animal there but its usually not a huge profit. It really all done for the common good of pets that need homes.
It's pretty simple how it works. A rescue group will contact the pet store and ask to hold the adoption day and the pet store will set up a space for the volunteers and the animals. The rescue group is responsible for running it from that point on and the pet store really isn't involved other than giving them the space. There is no money exchanged between the two parties at all. If a pet store asks for money to hold an adoption day then I would definitely find another pet store.