For a "prompt flexibility" service to switch something interruptible off for an unscheduled period of minutes whenever it sees a shortage in electricty supply, there needs to be some sort of signal which tells it to do so.
National grid frequency is a uk-wide indication of scarcity. It used to be The way to see that all the heavy steam turbines were running a bit slow due to high and rising electricity demand. National Grid Frequency was historicly one of the signals indicating time to fire up a diesel backup generator or open a big steam valve at a huge power station. Instead, that same signal could bump the thermostat down a degree a swimming pools and household immersion heaters nationwide, to get out of electricity supply shortfall without using fossil fuels to do so. When there next is surplus of generation (preferably renewables), everything speeds up to normal 50.00Hz, and when it goes over 50.05 Hz which is quite fast enough, bump all the thermostats back up a degree. In that way interruptible loads would preferentially use electricity during minutes when there is surplus generation, which can be from having an excess of intermittent renewables.
It takes me 21milliseconds to spot a really terrible F condition with precision of 0.1Hz, 105milliseconds to get better precision, and so national F tells me the most urgent tenth of a second over the past minute in which something ought to be switched on or off. Since reading a website listing regional status would take longer than that, the regional website should publish some numbers which I collect from time to time, and F should trigger when a switching event happens.
I've not been too focussed on only-F triggered switching because it is a national state indicator, and I'd expect that a regional server with a number and forecast of regional surplus or scarcity ought to be more useful to balance nearby distribution than a national-only number like F.
Now for Han and the luvvies who don't do star-trekkie-speak and don't need to follow what I wrote above, I here simplify to what I think could have been feasible at the technology level of 1982 and still has not been implemented. In Hull, there should be cheaper electricity while the wind blows on the turbines, and for Oxfordshire there should be cheaper electricity while there is sun on the solar panels.
Back to Scottie in the Engine Room, events such as sun on solar panels at scale boost national F, which is a useful indicator of the best second in which to switch interruptible loads, with the regional servers recommending a scarcity number from which my smart house calculates a different threshold F appropriate to the forecasts and scarcity in the house and in the local region.