If you look on the seed packets they should give you an ideal germination temperature. I only use a heated propagator for things like tomatoes, peppers, marrows etc. If you're growing things like cabbages, onions etc they don't need as much heat (in fact lettuce won't germinate if the soil temperature is too high). About 7 degrees C will allow most hardy veg seeds to germinate well, so if there's no hard frost forecast an unheated greenhouse should do. Either that, or a windowsill in the house, and then outside as soon as they've shown their little faces.