[rjwilmsi]

Yes, I'm aware of the Agile Octopus tariff etc. but answered based on what >95% of UK retail consumers are doing.

From historical data, Octopus would also pass on a negative spot price to customers on that tariff so consumers could sometimes get paid to use electricity.

As you say I'd imagine that certain customers with high flexibility, maybe/particularly those with some battery storage (or an EV to charge), could make huge savings.

I don't know what sort of margin Octopus apply on that tariff, I expect it's a bit higher than the 3 p / kWh you suggest (when normal tariffs would average something like 15 p / kWh difference between average wholesale and retail).

[tialaramex]

Yes, I don't know what their margins are, including I don't know whether they recover their desired margins from all supply evenly (e.g. 15p per kWh all the time), proportionally (maybe 30p per kWh when wholesale electricity is expensive, 0p per kWh when it's very cheap) or according to some crazy formula, this strikes me as "secret sauce" for such a business.

For a huge fraction (maybe 95% for all I know) of UK retail consumers they're still with their legacy incumbent supplier, even though those deals are usually more expensive and the service is no better, "privatization" was largely a waste of everybody's time and money. But at least in principle they could all choose Octopus.