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Stoke Rochford Hall, Near Grantham, Lincolnshire
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It depends on which specific component you are referring to:
The First Allied Airborne Army was headquartered at Sunninghill Park under the command of Major General Louis Brereton.
The U.S. XVIII Airborne Corps was headquartered at Ogbourne St. George under Major General Ridgeway.
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.