We'll be doing exactly the same here, Rowland, but we do have a small heater in the GH to obviate frost. They aren't covered in any way.
We lost about a quarter of our fuchsias last year, but oddly enough, most of the geraniums survived, and we could get them out for a display before the plug plants got going!
I hope we can do the same this year, as we invested in a lot of new fuchsias this year, so fingers crossed for a better winter and spring next year!
Never thought about humidity in polytunnels Mum, it's an interesting take! GHs are usually pretty draughty - well ours is!