More then 10 feet apart

Five miles, in my opinion!

There is no defined planting distance in UK law - as soon as the branches start sharing space it is a hedge rather than a row of trees.
The high hedges legislation defines a hedge as "a line of two or more trees or shrubs"
So if you don't want your trees to become a hedge then you need to plant them at least as far apart as the mature spread of the tree ie trees with a mature spread of 20ft need to be at least 20ft from any other tree or shrub.
5 miles suggested by Bulbaholic is a good start.
What amazes me is that people still buy the things, and even worse people who should know better still sell them!

Thuja's so much better.......

The selling point has always been quick growth and evergreen habit. As usual, people are thinking a year or two into the future, and not what problems they will have after the first three years. Maybe eventually the presence of such "hedges" will wind up reducing property values.

Thank you all for your replies. My question wasn't adequate for the information I needed so I've asked a new one.