@2ManyBoxes @MJS There are two electricity grids serving the United Kingdom; The GB National Grid, managed by National Grid plc which covers Britain and outlying islands, and the Irish National Grid, managed by the Irish company EirGrid, which handles power system management for Northern Ireland.
A DC interconnect between NI and Scotland, and a second between Dublin and Wales handle power transfer, but these are a relatively new development, and prior to 2000, energy supply was independently managed.
France and Germany are both part of the Continental European Synchronous Area, which spans 24 countries (including Turkey, Morocco, Algeria and Tunisia) and has over 400 million metering points (the largest in the world) - Scandinavia and the Baltic States have their own separate grids.
But have a look at Japan: because electricity remained a private business in Japan with little unification post-war, the country has
ten separate synchronous areas today, three of which operate at 50 Hz, while the remainder operate at 60 Hz.
To slowly swing back on-topic, interconnection between Europe and North Africa is a major goal to improve the viability of wind energy: because Europe receives most of its wind in Summer, while North Africa is windy in Winter, but electricity demand is opposite that pattern in both areas (cooling is the major power consumer in Africa, while heating is the big load in Europe), interconnection allows surplus wind power to be pushed into the grid for use, rather than foregone.