Does the Thom BAR report include all the 1,000 or so stone circles in the British Isles George or just a selected or maybe even random amount from various parts?
I kept to just Bodmin Moor because that is what I know and don’t have to rely on what others have said/claimed. Often some/many of those 1,000 circles have never been visited by the claimants who have simply nicked the info out of other publications. I don’t suspect that is the case with the report however.
Also it depends on what is considered as not being truly circular. You can mark out a circle (with a line from your central point) then set the internal face of the orthostats to it. Alternatively, you can set the outside of the circle to that line, or indeed the centre of the stone. In a large circle you hardly notice any of these variations, but in a much smaller circle you do if your selection of stones vary from absurdly small to large (as in some of our Bodmin circles). A small circle that has been set up to follow a line on its internal face but has a very large stone or two in its setting, looks way out of circularity at first sight.
Then there are the circles that have been re-erected by unqualified people (Goodaver as one example on Smith’s Moor which was put back together by a local farmer, farm labourers and locals).

(Wikki have this to say about it as I wasn’t about then)
The circle has a diameter of 32.3 metres (106 ft) (almost exactly to thirty nine Megalithic Yards) and consists of twenty three standing stones and one recumbent that has disappeared over the last century, they were originally thought to number twenty eight. The stones are well presented situated near the top of Shephard's Hill at an altitude of 306 metres (1,004 ft). The stones are generally rectangular and measure between 0.6 metres (2.0 ft) and 1.6 metres (5.2 ft) high by 1.45 feet (0.44 m) wide. They are spaced approximately 3.7 metres (12 ft) apart, slightly more irregular but in line with other big Bodmin Moor circles. Reverend A. H. Malan discovered the circle in 1906 when only three stones remained standing. Local farmers supplied workers to erect the fallen stones in their current position and it is clear that several stones were inverted, spaced incorrectly with some reversed faces and possibly in the wrong places.

I suspect many like this have been re-erected as perfect circles when they may not have originally been so. Once a circle has completely fallen and been that way for donkey’s years in which time some stones may have been dragged away for gate posts etc, it is difficult to assess exactly where the missing stones stood if you were an amateur or without our modern-day methods of detection. Again on Bodmin Moor, because the ground is generally so peaty and soggy the socket holes are completely silted up and covered with turf and that bloody creeping chain-mail type gorse until they are lost.