I've installed one at my allotment. :)
Because a bat's roost has full protection under the Wildlife and Countryside Act - whether or not a bat is currently using it - it will make it difficult to do anything with the tree it's in - but then if there are bats in the area you'd always have to consider them anyway. It doesn't mean you couldn't get rid of a tree with a bat box, just that you'd need a bat survey to say that the roost could go.