There was a thread on BCUK a year or two ago about pitching camp in the most public places possible. If I remember, you had to pitch a tarp or some sort of shelter, have a brew, take a photo and get out without being arrested.

There were some pretty good efforts but it died a death for some reason. Now, the motorway roundabout does seem like a good spot although, as you say, access could be somewhat hairy.

Martin