There was a supply request form that wass filled out when the original supply was connected. That form included a box for the "maximum power requirement kVA". Someone must have filled it out initially and that's all the power you're entitled to. The original cost of the supply would have been higher if the power request were higher, so it's very likely that the originator asked for the lowest reasonable power.
Now, if you need more power than was originally asked for, you will have to pay the cost. That'll be more that the extra it would have cost originally. The DNO's kWh charges (to your supplier) are regulated and nowadays do not include anything to cover the connection cost.
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