We are staying I London during the 17th-23rd of Sept. Is the palace or parliament open to the public?
The State Rooms of Buckingham Palace are usually open from the end of July thru September. This year they closed on Oct 2.
Altho I did not visit this year, I have done so on several previous visits. I enjoy it immensely! I usually get the Royal Day Out ticket which covers admission to the State Rooms, the Queen's Gallery and the Royal Mews. The Queen's Gallery has a specific exhibition each year so I recommend adding this ONLY if the exhibit is of interest to you. Last year it was on palace gardens which I found very interesting. This year the exhibit didn't intrigue me as much so I skipped it.
Do watch the above link for exact opening times for 2017 and get your ticket in advance.
The Palace should be but I am sure you can check the dates as you get closer. The palace is open to the public when the Queen goes to Scotland each summer. When we you there last month (mid-Sep) it was open but don't know the exact dates this year. Don't know about parliament.
17th-23rd of Sept
Lacking time travel, I presume you meant 2017. I agree with Pam, the palace is well worth visiting when the State Rooms are open. It was a highlight of my recent visit. Amazing rooms, art, furniture, places where history happened. A good free audio guide that could have been more informative about specific items, and helpful staff along the way who seemed to appreciate my questions.
Watch this website for dates, and order tickets in advance for a specific time. There were plenty of choices a month ahead. You can print the voucher to bring to the Palace. Then you can tell your friends you have an appointment at Buckingham Palace for such-and-such a date, and watch them look envious or nauseous.
I believe the Houses of Parliament are also open for tours in September, with timed tickets. We picked ours up the same day, as I recall -- this was some years ago. Also fascinating and historic, a guided tour rather than wandering through with audio guide. But I understand that the building is going to be closed soon for major renovations -- it might not even be open next September. Watch this website.
I toured Buckingham Palace this past summer when I visited England. I absolutely loved it. Just seeing the history and the beauty of the Palace. Knowing that I was walking on the same floors that head of State have walked on before, I was awestruck. I definitely recommend it.