You can't currently change supplier every half an hour. But with the right technology your supplier can change price every half an hour.
The only UK supplier with a half-hourly tariff is Octopus Energy.
- https://octopus.energy/agile/ changes price every half an hour, tracking wholesale prices.
- https://octopus.energy/outgoing/ works like Agile but also gives you half hourly export pricing. So you can sell excess energy from solar or battery storage back to the grid when the price is highest.
(Disclamer: I work for Octopus Energy)
OP here. That looks really interesting. Especially the V2G stuff. I've only just started with Bulb, but I'll certainly consider switching to you in future!
I think it's safe to assume that the comparison flat tariff won't be available that way (or, at least, at that price.)
(I'm not sure this functionality is actually available yet - most suppliers are still barely rolling out SMETS 2 meters - the ones that aren't supplier-specific.)