Your DNO is likely to be Scottish Power but with over 100A it needs you to request exactly what you want and the price can easy become silly.
It is unlikely that most houses will use more than 60A or at least unlikely they will blow a 60A fuse.
Supplies are normally 60, 80 or 100A most around the 80A and with 100A one can draw 160A for short time without blowing the fuse although it tends to weaken it and in the end it will blow.
If the supply is split for metering then likely they will down grade both fuses and in real terms only time it becomes a problem if off peak heating is used.
However since much depends on what the DNO has already to guess on cost is pointless. If to supply extra would mean they need to change transformer or cables then cost could be very high. But if all they need to do is fit a second fuse then much lower and only option is to talk to your supplier.
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