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