BT TV boxes will work without a Freeview aerial connected, but you'll be missing out on lots of channels and some On Demand services.
You won't be able to watch or record any Freeview channels like BBC, ITV, Channel 4 or Channel 5. You'll need to connect your aerial and scan your channels in order to receive all of your On Demand services.
Related help
Bad quality or missing Freeview channels on BT TV >