Saving tubers

As Pete said: no rush with Dahlias. I often don't lift mine until well into December (if it is mild and not excessively wet). Only reason for lifting Dahlias is:

Just in case the winter is really hard - and might kill them
To get them to start earlier in the Spring
To propagate from them (in the Spring).

so in future years you might just dig up some (for "insurance") and leave the rest in the ground.

When I have Dahlias with very small tubers (purchased late in the season) I lift them (now-ish) and pot them up to keep them growing; those I try to keep growing right through the winter so they they don't go dormant. Generally I get to late January or early February before they give up, but they don't really go dormant so they start again easily / early. Cold greenhouse won't be warm enough for this trick, although a cold conservatory (i.e. if it keeps above 0C) would be.

Ditto with small Canna plants.