It will take longer to reach thermostat opening temperature in cold weather. Engine and fluids are colder and you will be using the heater more. I suppose if speed is fairly low (generally not above 50mph) and heater temp is set to high and fan heater fan is running at high speed, it may be possible that the thermostat doesn't open.
Generally my JTS thermostat takes around 5 miles to open but half of that is 70mph on the dual carriageway.
On an A road when driving perhaps 50mph when the outside temperature is freezing, I've seen it take 10 miles.
A few years ago I did a 30 mile journey when the temperature was -20 degrees and I had to drive faster than was prudent to try to get the gauge to almost 70 degrees. Needless to say, the thermostat never opened and the radiator was cold enough to stick skin to at the end of the journey.
The heater matrix is a small radiator. The more air passing over it to try to heat the cabin, the more it will cool the coolant.
From my point of view congestion and SMART roads may only serve to slow traffic so much that engines stay cold and stay on cold start enrichment for the whole journey which equals greater emissions.
Not very smart.
Next time it seems to be staying cold, after about 5 miles, pull over, open the bonnet and feel the hoses. One eater hose should be hot, the other fairly hot but the radiator top hose should be almost stone cold if the thermostat hasn't opened.
Radiator top hose same temperature as heater hoses = defective thermostat.