Almost any plant, provided it is woody and does not mind a bit of competition and being cut back occasionally can be used in a country hedge. But that does not make it a great hedging plant. I have, honestly, seen a hedge made entirely from Ash. Interesting idea, but utterly useless.
Here are my top ten country hedging plants:
We went to stay with the in-laws this week-end. They live in Leicestershire, just outside Market Harborough. My father in law is a natural planter - one of those people who looks on themselves as being a custodian (rather than an owner) of the landscape. He is eighty now, has spent the last 50 years living in the same house and has used most of those years to plant trees, sometimes in large numbers. Almost exactly 10 years ago, we planted a copse - I provided the trees and understory (the smaller shrubs that grow beneath), and the hedging that grows round it. He provided about half an acre. Ten years sounds like a long time, but it passes in the flick of an eye. Bill and I walked up to the copse today and marvelled at just how BIG, the little bare root whips of 1997/98 had got. There are thirty footers amongst them. Some have been killed by the competition from their neighbours, but what was a corner of a field is now an established wood, teeming with wildlife. Once planted, there was no maintenance barring an annual slash of weeds for the first three years and then a bit of lopping of branches that got in the way. 
Pictures tell it all I hope, but there is an amazing thrill in seeing something young and vibrant and immensely powerful that, mankind willing, will be standing long after my children’s childrens children are forgotten…. (by the way that last was a comment that was made by one of my children)
We have had a few complaints this summer from people who bought trees from us that died. When you sell as many as we do, this happens sometimes (and we replace them under guarantee so all is not lost, dear customer!)
Well sort of. We sell a lot of plants each year and having only a few hundred of something popular tens to cause more trouble than it is worth - you sell out before the season has started and only end up upsetting people.
Well, we have been building our stock of blueberries to the point where we will at least last the month of November. Blueberries, of course, as “in” - they are amazingly healthy and given acid soil (post on the subject coming) they are wonderfully easy to grow.