I had to cycle down a busy section of the A27 near portsmouth last week. Only one junction, but looking at the map, it was the only way to get across the water. Once we were on it we realised there was in fact a pathway the other side of a row of bushes and a ditch. However, it wasn't well signposted and once we were on the carriageway we couldn't get back over to it. Technically there's nothing to stop you cycling on that stretch of the A27, but it was a most unpleasant experience. You wouldn't find me on a motorway, but if the guy perhaps wasn't armed with a map of all the crummy backroads the law says he should have taken, and the only signpost for the town he wanted to get to pointed to the motorway which is quite likely, then you can almost understand it. Presumably since it was the middle of the night he rather wanted to get home, than mess about on country lanes (which half the time are more dangerous than the motorway with lack of visibility, poor surfaces, no street lighting and unenforced speed limits).