Excellent Site!

What are the lines that cross Scotland ? At time of writing they are red where as other lines further south are green.

I know of some on shore wind up near the Rochdale area too. Does it mean they are offline if they just appear as black dots on the map?

Those are NESO (system operator) grid boundaries. The colours represent the forecast flow of energy over each boundary in relation to the capacity of each boundary. Green means lots of extra capacity, red means over capacity. When a boundary exceed capacity it's likely that this constraint will result in wind farms being turned off to reduce output "behind" the constraint.

The black dots are wind farms and other power assets that don't have any generation data. This is usually because they aren't connected to the transmission system, not that they aren't actually outputting. Or to put it another way, I only have data on power assets connected directly to the main transmission grid.