Wholesale rates aren't fixed. I think you may be referring to where the government has invested a significant amount into a power station and so agrees a strike

Wholesale rates aren't fixed. I think you may be referring to where the government has invested a significant amount into a power station and so agrees a strike price/contract for difference setup for an agreed number of years, which were often woefully unfair to the taxpayer. Though, current prices even start to make Hinkley Point look like a good deal.