It's easier to beg forgiveness than to ask permission.
Don't try that at Duxford. There is often a lot going on at Duxford and the staff also like to know that you have had a full briefing before you set off. If you don't ask BEFORE you set off the only way you will land there is in a dire emergency. Having said all that, it's a great place to visit with two well maintained runways and other facilities and all the staff I have met have been really friendly and helpful.
