This is a bit further afield than usual, but I've heard good reports about it. Judging by last years prices it should be about £15 plus travel costs. (Junction 8/8A of the M40 or train/bus via Oxford).
The house and gardens are worthy subjects but there will be some dancers/musicians in the open air theatre to draw as well as workshops to take part in. Not to mention the Art Materials & Publications Tent if you need some inspiration.
Find out more at: http://artinaction.org.uk/Art-In-Action-2013