I put mine in for about 6 weeks last winter. They had about 8ft by 14ft Perches, raised platform, ladders, deep straw litter. They had the same again outside (concrete floor, half wall, then chain link, and covered in weeds) but chose to stay inside. Some did keep laying. Think had10 birds.
It was good to have a break from rushing home to shut them in all the time (and having to let them out so early in morning that it was dark and then worrying about fox).And no mud to trudge through all the time.
Do you have a spare shed? I don't know what the legal limit for min space is, but I think if you can get away with flocks of 15 as you suggest, you can squeeze space per bird, especially providing interest like different levels, veg hung up, several feeders/waterers, etc. and it's only for a couple of months (whereas when I see two hens in a little run in a garden I can feel sorry for them).