Hello Herdis! I live in north Norway (70 degrees north) and am very interested in getting made a pit greenhouse. We also have the Gulf Stream, but probably have greater variations of temperature than you have. We also have 2 months of "Polar night" (twilight) with corresponding midnight sun for 2 months... But even though we can't grow during winter, we can greatly extend our growing season by having a pit greenhouse. We have even been thinking of the possibility of putting up a little windmill to power extra heat if needed...
I also find it very difficult to find useable information for us living in a bit "extreme" conditions... Hope this forum can help us...