That they are doing recce in Windsor Castle means they've been given permission to shoot on the grounds. Obviously, someone close to the production had been to Windsor Castle, thought it perfectly matched the setting they needed, and had the connections to seek and be granted permission. It would be a very special privilege given to them to film Maleficent 2 there. Per reports, the only other non-documentary that was given permission to shoot in the castle was the TV mini-series Victoria & Albert. Four others were all documentaries on the royal family. Maleficent 2 would be the first that has no royal connection.
They're still planning how to film at the site Not yet close to being ready to start filming -- at least not in this site.
Angelina Jolie Wows Viewers as She Expertly Pilots a Plane over Namibia https://t.co/FEr2K8GPBL— People (@people) April 19, 2018