What is the point of the holding point anyway, is it to restrict the number of people inside the building?
Yes, I believe the corridor is meant to be kept clear at all times. Back when the ride was marketed more as a thrill ride than a family ride and the station played the music much louder, more people were let into the station but they never had them queing in the tunnel. They also had at least 4 crew checking restraints and they would run the length of the station checking them.
The gate is also something that has been there since the Tussauds days. There also used to be a tree full of chewing gum just before the gate too.