Why didn Buckingham Palace fly the Union Jack until recently?
The Palace is the Queen’s personal residence and as such she may fly any flags she wishes. Up until now, the Palace has flown only the Royal Standard, and only then when the Queen is in residence. It was decided that the Palace (and the Queen’s other residences, except for Windsor Castle, which is technically a British fort and thus flew the Union Jack already) would fly the Union Jack in order to express a greater measure of solidarity with the people of Britain.