I don't mean to be rude, but have you been living under a rock? You're living in London, UK, which is FILLED with yew hedges. I think they're the most popular one over there.
There are alot of plants that naturally grow into trees that can be trained as hedges: yew (duhh), hornbeam, beech, maple, hawthorn, arborvitae etc...
You're living in London, UK, which is FILLED with yew hedges. I think they're the most popular one over there. 
