That's the White Drawing Room at BP. It doesn't house items from the Royal Collection per se in the sense that the room is some sort of repository, although it contains items from the Royal Collection. Interestingly, the mirror to the left of Queen Alexandra's portrait, disguises a door from which the Queen apparently likes to emerge to surprise her guests.
If you haven't visited Buckingham Palace, it's worth doing so. Parts of it are surprisingly small and it's not as grand as I thought it would be. Nevertheless, it's fascinating and you don't have the sense of being hurried along.