It’s a subsidy when the state ends up paying the difference between the guaranteed price and the average market price.

I guess another way would be fixed prices for all nuclear and renewables since they have fixed costs anyway. The thing is, the system already works that way to an extent, as there are a lot of guaranteed prices in renewables and some in nuclear. As far as I can tell, these subsidies are why the market price sometimes goes negative when solar or wind produce a lot: the subsidized sellers don’t care, they still receive their guaranteed income based on the quantity of energy they inject into the grid. It seems to me that without these subsidies the market prices would always be high, thus better reflecting the true cost of electricity.

Setting the electricity market price based on the most expensive source at any given time is pretty much universal in Europe. It’s an EU thing, not a specificity of the UK.

Changing this rule would reduce the income and profits of climate friendly electricity providers, thus penalizing the investors who tried to do the right thing.