On a new build, you have to arrange to get a new connection with a power company. Smaller suppliers don't offer this, so
you'll need to talk to one of the bigger players, like Contact or Meridian (you can switch back to your preferred supplier later).
You can do this over the phone, and then pass
on the details to your builder so the electrican can arrange to make the actual connection and install
a builder's cabinet. You will typically need to meet the costs of doing this, as well as paying for power during the construction.