Posted: 13/01/2015 at 11:51
I am in Cornwall and leave many dahlias in the ground over winter. And they survive. However, my soil is sandy loam....free draining.
If you think the ground is soaked and will get even wetter ...and colder...you could lift a couple and pot up if the tubers are sound. If they are sound it will also give you an idea as to how the remaining tubers are. I would be inclined to dig them all up, even now, if you have the room to keep them dry and frost free.
I would pot up using dry compost ....do you have a greenhouse or porch, etc.?