Canna rhizomes!

You're supposed to dig up the rhizome or corm, cut off the top growth, and repot in sand and keep in a frost free place over winter, repot into proper compost early next spring and start it growing indoors or somewhere bright but not freezing, then move outside when the weather warms up.
In practice, its boring - I have left a couple out on my balcony (south facing) against the wall and they have grown again the following year, but then its a very sheltered spot there.
The other thing is, you'll probably find the corm is 3 times as big as it was when you first planted it - you can split the corms into chunks and replant them separately.

Good drainage is more important to overwintering than the low temperatures. Here we had minus 21 for a few days last winter, and yet the cannas in the flowerbeds have all come back.
The ones planted near the pond and river in damp soil all perished!
I'd leave yours in place and cover the spot with a few inches of straw when severe frost is threatened. Remove the straw when the danger is past. Then carefully excavate the corms in mid spring to divide up.
If your canna has set seed, it's worth keeping this and sowing in spring as you can often get plants that flower in the first year if sown early. (though with named varieties they may not be exactly the same.)

In the western temperate climates, Cannas are still recognised by cultivar name, in order to sustain a thriving industry.
In reality, there are so many hundreds of thousands of different cultivars, that naming has now become redundant, particularly in Asia.
Here, you will find Cannas growing happily at altitude, with nightly freezing temperatures all year round.
Conversely, permanent pond planting is ok too, but in WARM water.
The rhizomes dislike cold at the roots, particularly with dampness, for inescapable lengths of time.

If you are South of Watford, I'd leave them in the garden

its in a pot peeps, so gotta store, me thinks!