Having just paid a bill to Supply UK for hydrant use in the Thames Water area and Yorkshire you aren't seeing the true picture.
We have used standpipes in Yorkshire, Supply UK but separate from the Thames account, Supply UK. You cannot hire a hydrant in Yorkshire and use it in the Thames area.
Supply UK in Yorkshire for up to 20m3 pr day was about £40.00 per day (that is including the standpipe hire).
For Supply UK Thames Water account it was about £40.00 per day plus hydrant hire - 3 days came in about £220.00.
Rob