Wow, so nice to read your question. We have an identical situation here on Mid-Coast Maine. Generally, I leave the hen house door open if the night time temps are going to stay in high 20s or more (not sure how that equates to C?). They get locked in if temps are going down in low 20's and below. A bit confused though, your location is listed as Hawaii but you say you are in the U.K.?
I am new at this too but it would seem they are fine with going in and out of their hen house at will, which is in a very predator (I hope) secure pen.
There's one thing though, they will get up on their pen roosts once it gets dark and pretty much stay put until dawn. So, far no frost bite so I hope leaving them like that is okay. I do pick up their feed once they have settled in for the night cause I don't want to tempt any other critters from getting in to their feed.
Certainly, it's much easier to have them decide when they want in to the hen house and their laying box. Makes my life a bit easier too, as you note dawn keeps coming around way too early!