> National Grid pays a lot of money for "frequency response" to ensure the grid remains balanced

In the US frequency response is mostly Nat Gas powered. There are new projects that are battery based, but another way to achieve this is on the demand side, by requesting non critical loads to turn off or throttle themselves when the frequency drops too low. I think this is being done at an industrial scale (i.e aluminium smelters) but it is possible on the domestic scale also with EVs electric tank water heaters, etc.

Going forward, given the intermittency of renewable energy, these methods (primarily storage) will be more important as the energy supply becomes more variable.