If Parliament is on your "must-see" list, and you really want a tour in English, then I would advise booking in advance. For me, it was a not-to-be-missed sight. Mid-October English tours were selling out in mid-September, morning tours selling out soonest. There are some same-day tickets made available, but I wouldn't rely on those if you really want to go.
You can go to the booking engine and easily see how many tours still have English (or other) spaces available - 30 days in advance. For example, I can see that this Monday, the morning tours in English are sold out; the afternoon tours still have availability.
With a little more effort, you can find out about how many tickets remain for a given date/time/language by entering different quantities to purchase and see how many you are able to add to your cart.