I've definitely seen packaged pre-sliced sourdough bread there, but I can't recall if I've seen fresh bakery sourdough which I suspect is what you are looking for. An example of the packaged stuff available on the island (from SXM Les Halles, a grocery delivery service on the island):
https://www.sxmleshalles.com/en/bread/4357-sourdough-bread-22-oz.html

I'm not sure whether marmite is easy to find on the island either.