Q: It's winter and my house is cold. Should I use a heating mat to warm it to 25°C?
A: While a heat mat will warm the kit, placing the mat directly under the kit will give direct heat, which will dry the bottom and be too hot, killing the mycelium.
The easiest option is to store the kit in the airing cupboard, where
the boiler is located. Don't store it directly on top of the boiler, but
in the same space. Usually the temperature is about right.
An alternative is a Heat Bomb set up. The most simple incubator is a heat bomb, made with a cleaned cooler box and a 2 litre drinks bottle. Boil a kettle of water, mix 1 litre of boiling water with 1 litre of cold water. The water is now around 40°C. Fill the empty water bottle with this and place it in the cooler box. It will keep the area inside relatively warm. It won't dry out the substrate or kill the mycelium. It will need to be replaced regularly and so requires manual intervention. There are more advanced heat bombs than this, but it's a decent start. A thermometer is highly recommended in this case.
A more advanced one involves the use of a small aquarium heater and heating the water bottle to 26°C. This water bottle will keep the incubator or cooler box ambient temperature stable around where you need it.