I'll add another comment about the amount of heat we need to warm a car.
I've used an electric fan heater (for many years) to thaw out my cars on frosty days.
I run a 2kW heater and it takes about 30 minutes to thaw all windows and put a reasonable amount of warmth in the cabin. That's at around -2C ... in a 3-series touring. It uses a Kilowatt's worth of energy to do the job! Get it any colder and that increases dramatically. 5-series touring takes another 15 minutes to do a similar job as in the 3-series.
So any idea a small amount of battery drain, say 'one chain' in the PTC element will do much good, over a short amount of time, forget it as a serious way to preheat the car.
You can see why BMW doesn't even entertain using the PTC element as a pre-heater. You need the diesel fuelled pre-heater to do a fast job, as they can give out a few kilowatts quite fast. But as said, not an option on the UK 3-series models.