The last 2 times I had 8:30a things going on in MK, they were letting people in at 8am.
The one time I had an 8am tour, they still let people in at 8am - I even asked if I would be able to enter earlier because the tour was supposed to start at 8am and was told they wouldn't let people in until then. Stressed me out but I was ok.
So I'm guessing if you arrive between 8am and your reservation time, they'll let you in if you go through the breakfast turnstyle.