I am getting a small unheated greenhouse for next years plants.

For tomatoes and similar plants then the greenhouse should be in the sunniest part of the garden with the ridge running east-west.
Alpine plants do not like to be scorched so we have ours in semi-shade with the ridge in a north-south direction.

Before you buy the small greenhouse, look at the space you have, and get a larger one. In my experience I've never found anyone who has said their greenhouse is too large, and hundreds who've said the reverse. Foot for foot, you get more value in food, plants etc from a greenhouse than any other part of your garden, and it's somewhere you can go to work, sowing seeds, potting on etc. (or drinking a glass of wine) no matter what the weather outside.