I believe the temperature at which regen starts to be limited is around 10C (at least for Model S). Note that it's the battery temperature that matters, and the battery has large thermal mass, so if the battery has soaked down to a low temperature overnight it may be much colder than the air temperature when you start to drive away. The temperature for optimum efficiency is much higher than this, so in winter driving it can take a very long time to get there - typically the 2nd half of a long trip will show better efficiency than the 1st half even if you think you've been cruising at uniform speed.
OTOH, the temperature for regen to be totally disabled is much lower (around zero I think, though less sure about that) and it relatively quickly gets warm enough to allow some regen - primarily because it runs the battery heater in those conditions.
If you go about preheating the battery sufficiently before setting off, does the car typically then remain warm with regen available until you reach your destination?
In UK weather, it's very unusual for things to get worse while you are driving. However, it's hard to preheat sufficiently to completely remove the regen limit, and if you drive off with (say) half the normal regen it can take a very long time to get to full regen if you are driving in city traffic - under those conditions it doesn't run the battery heater, so you are relying on motor waste heat and self-heating as current is drawn from the battery, both of which have minimal effect if you are just crawling along in traffic. Bombing down the motorway will remove the regen limit relatively quickly.
The answer to "why don't they make it easier to preheat to get rid of the regen limit?" is that there's no point - for a short trip the energy wasted in pre-heating would be vastly more than would have been regained by the extra regen, and for a long trip the waste heat will get you there soon enough for free. So the only real reason for wanting to do that much preheating is if you don't like the driving experience with partial regen.
don't think you can pre-heat the battery on an M3, AFAIK it uses heat from the motor to warm the battery in cold weather.
I don't think this is relevant. Both Model S/X and Model 3 use waste heat from the motor to heat the battery; Model S/X also have a separate battery heater (at least in classic versions), while Model 3 runs the motor in a deliberately inefficient mode to achieve the same effect.
You couldn't have no active battery heating mechanism at all, since you need to be able to charge in sub-zero conditions (where you can't charge at all without heating the battery). There's also no reason to believe the 3's motor-as-heater mechanism is less powerful or efficient - the amount of waste heat coming out of the motor at full bore is greater than the rating of the Model S separate heater so the liquid path has to be up to the job, and there's no difficulty running a motor of that type at 0% motor efficiency (= 100% heater efficiency) so the electrical path is better.
So while Model 3 is different in how the scheme is implemented, the net effect is the same.
I wouldn't be surprised if the Raven Model S has ditched the discrete battery heater in favour of the Model 3 scheme (given it has the Model 3 motor), but I haven't seen this reported one way or the other.
