The frequency aspect of a black start is presumably a bit easier in Europe because there's an interconnected synchronous grid so they can bootstrap it from France essentially.
It's far more problematic for the UK because all the interconnects are DC.
I was recently told by an electrical engineering lecturer that the black start plan here in Ireland is to use the DC interconnectors with the UK to provide startup power to a synchronous generator.
With the new wexford-wales interconnect that went live last month, and another one planned from Cork (?) to France things might be even easier in the near future I reckon.
With the DC interconnect, your DC to AC conversion equipment would need the capability to provide synchronized power to the generator you are trying to start. With the synchronized grid tie, your are pulling the generator into the running grid.
A synchronous interconnect provides not just a source of truth but also stabilizes your grid frequency. If you have an isolated grid you have to match generation to demand to keep the grid frequency stable. If you have a 1GW interconnect that means you can mismatch generation and demand by up to a gigawatt and still be fine. I imagine that makes for a much faster startup procedure
