The diameter of the Stonehenge sarsen circle is about 100 feet. Assuming a slope for the roof of about 40º, that would need timbers of about 70 feet in length. I don't think that's feasible with British trees, but I could be wrong.

It has been proposed that Stanton Drew circle had a roof, but it also had many more concentric circles which would allow the use of shorter timbers. http://www.cems.uwe.ac.uk/~lbull/stanton.html