Back in my early days I used a heating pad. Like for your back.
It would be best not to have the plant sit right on it. Don't fold it either. The best place would be under the bottom floor like radiant floor heat. when the wood warms your cabinet will be perfect and your plant will love you.
If you can't put it under the cab stand it against the back wall either way you have control with a heating pad to adjust the temp.

You may not have overwatered but just too cold on the roots.
I'm in very similar climate straight across the pond. hope this helps.