The US runs three major grids: East, West, and Texas. There's interconnections, but capacity is limited. Sounds like the UK is similar here with the bottleneck between Scotland and southern England.

Within the US grids, there's really subgrids with interconnection and bottlenecks, too, but those interior bottlenecks aren't brought up as often as say overnight wind production in Texas being over local demand as well as interconnect capacity.

england and wales are run by national grid, who also have a huge us operation - scottish operations a bit less clear

National Grid ESO are the system operator for the GB grid.

National Grid Electricity Transmission operate the transmission network in England and Wales. The transmission network in the south of Scotland is operated by SP Transmission; in the north of Scotland, it's SSEN Transmission.

Even in the parts of the US that are unified grids, a kWh is not a kWh. Where you live determines how expensive your electricity is. Compare Cambridge, MA https://electricityrates.com/compare/electricity/02139/ with Philadelphia, PA https://electricityrates.com/compare/electricity/19101/. About twice as expensive in MA.

GP was (I think) talking about what the grid pays the plants, not what consumers pay the grid.

> A kwh is a kwh.

Start to finish, the OP is a detailed analysis of why this is not true.

Unfortunately there is no big copper plate in the ground connecting all of the UK. Transmission capacity between different parts of the grid is limited.