Are those gas plants not peakers? It seems wild to me there is 5gw of curtailment while there is 16gw of gas going. Surely the grid could use some more storage capacity.
Virtually none of the gas power in the UK is traditional "peakers" aka Open Cycle Gas Turbines. There's a lot of Combined Cycle Gas Turbines. Gas is much more expensive in the UK than, say, the US and so it doesn't make much sense to build OCGT because although they're cheap to build they are very inefficient so your bid prices will be really high.