You can take a fixed wing in - ONLY on certain days. I have been in during the Coys Festival. However the airfield is shut most of the time as the runways are in use for some events.
You need to call Silverstone Circuits, and seek permission. They will fax (did for me) the airfield layout etc. Sorry I dont have any numbers or contact with me today.
No landing fee you just have to pay entry for the event.
Or as the previous post, Turweston.....