I think that the roadway you can see in the photograph is the perimeter track of Old Sarum.
I have seen other pictures of the incident and that is definately a public road. A clue are the ploughing furrows visible under the snow behind the port wing. There would be a bit of a problem doing a 2.75 mile tow from Old Sarum to Boscombe. There were only a handful of runways that could take a B36 so your average road, then, would have collapsed under the strain; even if it was wide enough.