I'm far from an expert here but isn't that spot price rather than future deliveries? Few people pay for actual spot pricing because it can go the other way, and you want known pricing. You would have a forward contract to delivery gas at say 20p. This is a known price for operation and likely has profit baked in anyway. The excess is what we see now. They can't just switch off as they have a contract to fulfill, but the grid doesn't need the excess, therefore priced at a negative.