So as I now understand it, an IMC rating allows only reduced viz for a SVFR within the Channel Is zone, and full priveleges once you enter one of the airport zones.
This is in line with the information published in the Guernsey Flying Club journal, but is slightly more restrictive than a quick read of the geographic extent of the IMC priveleges, mainly because the Channel Is zone is class A.
Apparently it also reverts to French control after certain hours, perhaps after the island airports are closed anyway?