They do, agreements are typically in place with companies that use a lot of power but can live without it for a fair few hours.

aluminium production/smelting uses a colossal amount of electricity but they can shut down quickly for a few hours (as long as they don't let things get too cold and freeze which is expensive as it destroys equipment and takes a long time to restart - which is what happened in the ransomware attack against norsk, they had to switch to manual line processes to keep it moving).

For anyone interested the people responsible for monitoring our grid have a good website with a tonne of info.

https://www.nationalgrideso.com/