For each island I found the point from which the maximum distance to the sea is. These are the islands that have the longest maximum distances. Does that make sense? The minimum distance to the sea would be at the coast for all of these...which would be zero.
Thank you for that. I estimated the distances myself by drawing circles that could fit inside of the island. However, it was sometimes hard to judge where exactly the coast should begin. For example, if a wide river gradually narrows as it comes inland, at what point does the coast start? However I will change Britain to that measurement.