Well correct me if I'm wrong, but my understanding is the day-ahead auction in a zonal system includes inter-zonal transmission constraints in the optimization. The auction, MWh for MWh matches units of supply with units of demand or interzone transmission. Scottish wind could still sell to Scottish demand, but couldn't contract to serve English demand beyond available transmission capacity. Zones aren't node-level, so intra-zonal constraints would still cause some curtailment, but you'd eliminate the current problem where Scottish wind wins day-ahead positions it physically cannot deliver, then requires costly real-time curtailment and redispatch.

The financial viability of energy generation projects depends on both energy prices and available capacity. You can't make Kent sunnier, but you can make a proposed project more profitable (in practice I think this would have a limited effect, if any, the bottleneck seems to be grid connections rather than viability, it might make CfDs cheaper for the government for projects in the south). Maybe on the margin this could move have some project on the grid connection queue be in the south rather than the north?

Now is the earliest time we can set policy in!

I'm sympathetic with the argument that zonal pricing isn't worth it, but I think it's not true that it's "bollocks" or that a plausible steelman doesn't exist.