Might be worth making some polls if not sure on location?
Of the two you’ve listed, my vote is Knockholt which is about an hour less driving so more accessible for us not-organised-enough-to-have-our-own-south-west bunch! 
There may not be a monument there (that I know of) but am sure someone would bring a wing we can play chase with?
My birthday on the 21st so I’ll bring beer?