Buying plants from Supermarkets and the “sheds”

I'm often asked about where to buy plants: is it necessary, Clients ask me, to go to a proper Garden Centre, or can we take advantage of the very cheap plants which they sell at the “sheds” such as B&Q and Homebase and, increasingly, our local supermarkets? On the face of it, it seems like an excellent idea: they are close to home, you were going there anyway, the plants are cheap and look very…