Since 1837, Buckingham Palace has been the official royal residence. It was originally built in 1705 for the Duke of Buckingham. The royal family moved in when their residence, St. James’s Palace, was deemed insufficiently impressive.
Buckingham Palace remains a main tourist attraction. Unless you plan on going inside for a very limited tour that costs between £15-60, depending on the type of tour you want and the time of year you’re visiting, or watch the Changing of the Guard, it won’t take long to stand outside, snap a photo and be on your way.
One of the benefits of being Queen is that you get to fly your own flag, the Royal Standard, on whichever residence you’re occupying, on your car, on your plane (only while it’s on the ground) and on buildings you are visiting. The Royal Standard cannot be flown at half mast, even after the sovereign dies (the rationale being that there is always a live sovereign).
The Union Jack flies when the Queen is not in residence. That flag, incidentally, can be flown at half mast, as it was for Diana’s and the Queen Mother’s deaths.