In such a cruel world, It seems only right to provide a exceptionally safe home for the Prime Minister.  Not hat I like this one, but for all of them.  
What I mean is, since terrorism and advanced weapons are available, we could easily take down the building, use the stone, but re-create a better building created out of reinforced concrete and steel that will be good for 500 years and provide exceptional safety from rocket launchers or some other small weapon, or gun so that even in the open, unless someone hit a window exactly no harm would come to them.  It still could be exceptionally beautiful and have high ceilings and windows and great insulation.  just very strong and safe.,