I think it might be time to move on from the property and build something new - 24 Sussex is old, decrepit, and not really suitable for the task. It's just an old house.
Preserve 24 Sussex and turn it into a museum or something, but build an official residence that actually meets modern requirements.
One of the reasons I moved from Vancouver to Scotland was because I was sick of doing stonework on multimillion+ mansions. I work in Conservation here. It pays.....well it pays like shit. But I get to work on buildings with proper history. We, North Americans, have a tendency to just knock things down instead of maintaining them.
I think it's really important that we maintain significant structures. Personally, I agree with you. 24 Sussex should be relegated to a museum. It should be restored, maintained, but it's no longer suited to task.