I call them pill bugs.... also known as sow bugs. They eat decaying and dead plant matter. So I think they wait for leaves and such to rot and fall off plants.
We call them Wood Lice over here. They eat decaying wood, especially if it is damp. They live under stones,slabs, pots etc. My chickens go crazy over them. They don't seem to do any great harm in my garden but you might have a super breed on your side of the pond.