The Future! A postcode search best for checking FTTP (fibre to the premises) availability. And you'll need to do an address-based search if you want to place an order.
Pick from the list of addresses in the OpenReach database. If yours is not found you could try "unlisted" but be warned that accuracy will be rather hit and miss!
Warning: Legacy Technology! Use an existing telephone number to find out what broadband services can be delivered on your copper cable.