For Northern Europe at that time I would think that kale would be the best source. You can harvest fresh kale for most of the year. No idea how the vitamin c holds up when you dry it. I'm unsure if sea buckthorn was growing in Northern Europe at that time, but if it was it would be a near perfect source, save for the trouble of picking them. Today you could eat Rosehip too, but that plant is so invasive that it needs to be seriously kept at bay.