I have an empty flower bed that follows the semi circular bay window at the front of my house. It has been empty since we moved in, and just has stones and gravel at the bottom (and some weeds!). It is quite shallow, about 7 inches deep.
I want to put some flowers or small shrubs in it, but I would like advice for which soil I should buy from b&q, ie topsoil, compost, multi purpose compost etc etc.

Also I want to line it with plastic so it wont cause any damp issues to the bay window. Do I line the flower bed with plastic and poke holes in the bottom for drainage, or do I leave the plastic intact in a "U" shape?

Thank you