The incentive is to sell all 100 units for £infinity. The incentive for the seller to sell at a high price isn't relevant. They don't have the ability to act on it. If they did, they wouldn't stop at £10,000. I'd pick numbers like £1,000,000,000,000 at a minimum if I was a seller and could truly set my own price based on my incentives.
If the UK has banned all companies except one from supplying electricity then they will without doubt see eye watering prices. But that has nearly nothing to do with the pricing scheme they use and a lot to do with the one company part of the picture.
It hasn’t, howver the natural state for these companies is to merge and diversify to maximise their profits, and the cost of entry into electicity generation is not like launching a bbc backed startup.
