"....
From Sonoma County to Humboldt County and as far away as Australia, the Internet giant has already posted photographs taken on private property.

“It isn’t just a privacy issue; it is a trespassing issue with their own photos as evidence,” said Betty Webb, a Humboldt County resident.

Webb said Google drove up her private road and past two “No Trespassing” signs to photograph her property."

Link:
http://www.pressdemocrat.com/article/20080821/NEWS/10644/0/news07

and
http://arstechnica.com/news.ars/post/20080824-google-no-tresspassing-signs-wont-stop-street-view.html

In the UK, there's no law against driving past a sign that says "Private Property" or "No Trespassing". If it's an open road, you can drive on it.

It's different if you somehow get past a locked gate, or if the owner has previously ordered you off the land.