Can someone remind me of the name of the house and country estate in southern England where much of Operation Market Garden was planned. I have even been on a course there in the 1980s, but can't remember or find it online.
It depends on which specific component you are referring to:
Detailed planning for Operation market Garden in particular wad done primarily by British First Airborne Corps as it was a supercedence of an earlier and less ambitious coup de main - Operation Comet. Location of its headquarters escapes for the moment.