If you are going to go do not plan on driving, parking is not available. The DC Metro ("tube" in Britspeak) provides excellent service. Be aware that "Pentagon City" station is not "Pentagon" Station and if you get off at Pentagon City (a shopping mall etc) you will have a walk of about half a mile. I understand that there was a while after 9/11 when you had problems at the Pentagon station if not holding DD 1172, 1173, 1173-1 etc (ID's) but it should not be a problem now.
US citizens wanting a tour inside the Pentagon reserve in advance through their Congressman or Senator's offices. Not sure about non-US visitors but I would guess calling your Embassy would be a start. The tour is interesting as I recall. I went with my son during Gulf War One when wife was stationed there TDY. Entering the Pentagon if you don't work there is like security at an airport. Try not to carry camera, cell phone etc as they get antsy about these things. Also, leave the Webley at home
Also, if you do not know DC in September, plan to dress for very hot humid sticky miserable awful weather. (on nice days, some days are even worse)
I am not sure as to the ceremony being open. I seem to recall the ceremony dedicating the site and unveiling plans etc was limited to official participants and the families of those who died at the Pentagon plus those decorated for heroism for their actions that day. My guess is that it will be those groups at the dedication and open to the public later or the next day.
Also, if you are in DC on a Friday, try to get the the Friday evening "Retire the Colors" ceremony at USMC Headquarters (8th St and I St SE). There can be a long line. If you are active military you might speak to one of the Marines monitoring the line and ask if a British military ID would help. One time I was waiting in a long line there with the wife who had come straight from the office in USN whites. A marine GSGT saw us, stopped and said "Commander,and Sir please come with me" and we were escorted to front row seats. The Silent Drill Team is amazing to watch.