I'm hoping it has changed, but when I booked a guided tour of Parliament, my confirmation email didn't include the time. I found that to be a bit odd and I had to actually call them on that Saturday to find out what time I'd booked. Security was similar to an airport, without the surly staff. I had my USA passport with me, I think they checked it briefly as we entered.
Keep in mind that you're required to stand for most of the tour, I had my Parliament tour right after a lengthy London Walk, so I was pretty tired at the end. I think I got a lot more out of the guided tour than other people did that used the audio guide. However, I'm a politics and history junkie and I've been watching the UK Parliament on TV ever since they started broadcasting.