Highclere Castle if you are going to Stonehenge where Downton Abbey tv series was filmed. On the days it is open, just rock up, don't try to book. English Heritage may still do a tour where you wander around Stonehenge with a security guard in the evening.
PS. Highclere was the home of Lord Carnarvon an early motorist who had a bad accident and went to Egypt for the warmth and backed excavations that resulted in the 1920's fashions.
http://www.touregypt.net/featurestories/carnarvon.htm