Off the Beaten Path Recommendations/List for London

Need recommendations, this will be my fourth trip so looking to do more non-touristy things this time. Also willing to go one another day trip that wouldn't take too long on the train. Help greatly appreciated 🙂
  1. Tour the inside of Buckingham Palace
    It's never been open in the months that I went before so this is at top of list
  2. Highclere Castle
    Loved Downton Abbey so making a day trip here!
  3. Notting Hill
    Never made it there before... want to explore some of the shops