Comment by Reason077

> "why would this be necessary when the entirety of Great Britain is one synchronous grid?"

Because there are bottlenecks in capacity on the synchronous grid that restrict the amount of power that can be moved from north-to-south (or vice-versa).

It works out better/cheaper/easier to bypass those bottlenecks with efficient undersea HVDC links than to try and build more terrestrial AC transmission lines.