@catwithflowers thats an interesting dahlia challenge!
Because you've planted them late in the season, there may not be enough weeks of summer temps and natural daylight for them to come on this year (but you never know!). I would give them a good drench with plant food (I tend to use Tomorite as a good all-rounder as it's got seaweed and other nutrients). A couple of doses spread over the coming weeks may well given them sufficient boost.
then, to your point, you could try putting them in the greenhouse in their pots, with all foliage and flowers cut off, protected with bubble wrap during the coldest temps but I'd beware (as previously posted) rot is the enemy of tubers, but equally so is desiccation (shrivelling and drying out). Keep the compost/soil slightly on the dry side of moist (or should that be the moist side of dry
- they don't enjoy sitting in soggy or parched-dry compost when the temps are low, outside their growing season.
[Pallaver alert!] Last year I rinsed all mine off with the hose in October, let them dry off and then carefully stored them loosely wrapped in newspaper in ventilated crates (£3 in Wilco) and stacked them under the greenhouse staging where they stayed quite happily until March, when I started all over again (good success rate, I only lost 1 tuber out of 18) - I then added a further 20 from a B&Q sale so I've got my work cut out this autumn! It's time-consuming and a faff, but I quite enjoyed the experiment of seeing how many would survive. They're rewarding me now.