Don't get complacent - laurel, azaelias, rhodedendron, shiny crisp leaved plants are poisonous, foxglove, ragwort, yew trees are all poisonous quite instantly too, - can't see leylandi on my poisonous plants list though - maybe google it. I would have thought it was but I may be wrong. One of our sheep died this year from a poisonous plant even when the others had avoided it - don't think they won't eat them, if some of the plants have been cut and lie on the ground they seemingly taste less bitter (such as ragwort) and so they can eat them then - this is often when the plant is at its most deadly.
I would consider all evergreen plants to be avoided for sheep.