It's not that simple whataboutbob. Even if the crops do need help (and they really do this year!) it would be extremely rare for most arable farms in the UK to have widespread systems in place to irrigate crops as we usually have the opposite problem in this country.
Irrigation systems are hugely expensive and can't just be put in on a whim very easilvery, particularly in fields where crops are already growing.
To answer your original question though:
It would depend on the farm and what agreement they have with their local supplier if they use one.
Farms are essentially treated just like other businesses for water use in that different businesses with different needs have differen agreements with water companies.
Our farm has a metered supply, so we pay for every cubic metre of water we pay for. We don't use a huge amount of water, mainly just water troughs for animals and water for the farm house or to clean the cattle trailer out with a pressure washer.
Other farms, particularly high water users, may be on a flat yearly rate for up to a certain amount of water so they know how much they will pay each year.
Lots and lots of farms have their own water supply (e.g a borehole or natural spring) and therefore only pay the water company/ environment agency a minimal standing charge or groundwater charge. Most boreholes have an annual limit on how much can be taken from them, but it's usually pretty high.