This matches offical maps for Jersey (from 2009 at least), so seems to work well there.
But it leaves Guernsey mismatched, recent (2002) maps use a grid based on the <b>New Guernsey Grid</b>. Which also have tick-marks plotted around the edge, in an unspecified datam, probably ED50? So a grid can be drawn on the map by hand.
So the question is how to resolve this? With different possilbities, there is no 'natural answer' that seems to fit, so it seems to be 'pick one' - but which one?
... and then the question what about the other Islands? (Note: I haven't been able to get hold of mapping of Alderney, Sark, Herm & Jethou - so dont know what grid/s they use.)