Well done for wanting to have a party. I think I'm still coming to terms with that milestone!
We've been to a parties at Ben's Canteens, Earlsfield and Clapham Junction. Both should comfortably fit in 70-80, have a bar and don't charge for hire of the place for the night.
Not sure about disco stuff. I'm sure they could have the Dad's dancing routines, perhaps even John Travolta. You might need to explain the sort of dance moves you're planning ....
http://www.benscanteen.com/private-hire