And if A did try and sell his product at below market price, without expanding output, then trader C would buy as much as he possibly could and sell it on at, you've guessed it, £50.
I've explained this a couple of times already.
The supply of renewables and nuclear is fixed, it does not and cannot respond to the market price.
Gas is the marginal producer, it can respond to market conditions, the cost of gas determines the cost of generating electricity (from that source), which then determines the market price of all electricity.
Even if they could, there's no logical reason for renewables producers to sell their output at anything other than market price, because, as mentioned, their output is fixed.
And, as also mentioned, all electricity is alike.