Agree with
@NE Ohio John ! You've such a big pot, it should be fine. But it must be put in a totally frost free area. Whatever you do, don't cut the stem, just in case you cut the eyes as the eyes are where the new stems come up this year. I live in East London which is famous for its drought and heat. I normally leave them in the ground (dahlias are from Mexico, so they love the heat). As you live in possibly a rain-sodden area, rain is more a problem as tubers hate being stuck in wet soil which will rot them. So it's a good idea to put them in such a big pot in a frost free shed or even greenhouse. If you have a propagator, you could put the pot in 15c propagator and you should see new shoots coming up.
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I think I did this about a month ago. Also, if you put a thin polystyrene tray (eg a pizza backing sheet) underneath the pot, it will raise the temperature a bit, but if the roots grow through the tray, don't worry, just snip around the tray and plant the tubers with them. They won't harm them. However, if you're a purist, you may frown at the idea!
Good luck!