At St Pancras they don't let you in until 1 hour before departure. If you try to put your ticket in before then the turnstile refuses to open and it says "seek assistance". I am not sure how hard they enforce the minimum 30 minutes, I have never been that close in recent years!
Assuming they have the same 1 hour max time at Gare du Nord, then there is not much point in being there earlier, all you can do is sit and wait for the turnstile to open.
I question Sharon's statement "People who showed up one hour before were turned away.", I think you mean "People who showed up more than one hour before were turned away"
At Gare du Nord the sequence is: (1) Ticket turnstile, (2) French/Schengen exit control, (3) UK entrance control, (4) bag X-ray.
They have limited space at Gare du Nord, every time I go there are long queues for (2) and (3), with people in the wrong queue (EU/non-EU) because they can't see the signs from the back. A scrum.
There is also a lack of seating. They don't let you board the train until ~15 minutes before departure and there is not enough seats in the waiting area for a full train load of 902 people.