The great hall in Alexandra Palace is just that - a big space. It's mostly used for exhibitions and antiques fairs, stuff like that. The only times I've seen bands there they've not erected any seating, just a reasonable height stage.
It's a local authority run building so there will be strict restrictions on the number of tickets they're allowed to sell. It shouldn't be crammed with people.
I will say this, the last time I was there was for one of the antique fairs and it was absolutely freezing.
One other thing - I wouldn't drive there, parking's not great. There's a train station just down the road, called Alexandra Palace, a five minute walk away.
Enjoy the gig