For total generation, see:
http://www.ref.org.uk/fuel/index.php?tab=year&share=NThey have 268 million MWh, but that doesn't include solar and embedded wind, so I upped it a bit. I imagine I'm within a couple of percent of the actual number, which is more than good enough for these purposes. In any case, the number will be different in 2021, but we can't know that yet.
For my calculation, it doesn't matter whether the capacity auction is won by negative or positive capacity, as I'm looking at the overall cost to the electric power system of ensuring that "the lights stay on". So my average cost is the average cost to the electric power system per MWh it generates over the course of the year. The 1.5£ per MWh is 0.15p per kwh, so a percent or so of the end user's bill.
Of course you can argue about the exact cost, but the point is really to show that when people say "you don't know when the wind is going to blow, so the power it produces is worthless, because you have to back it up", that's not true, as there's actually a low, measurable cost of guaranteeing supply. And of course many people argue that these capacity auctions are unnecessary, as we'd have enough available capacity without them. So in some ways it's a worst case scenario cost. And obviously it isn't just backing up renewables.