Hi Darren,
Butane doesn't ever freeze in the UK, what it doesn't do is boil below 0C.
Propane has the advantage that it boils at minus 40C, so it will generate gas at much lower temperatures. But propane has about 10% less heat energy in the gas - so if you do change expect lower performance from your appliances.
Do not insulate the gas bottles in an attempt to keep the heat in - when you draw off gas, some of the liquid needs to boil to replace the used gas - and this sucks in heat from the outside.
If you are using butane and the gas doesn't flow so well, dribbling a jug of warm water (electric kettle) will get the gas flowing again.
The coldest we've used butane was with a minus 5 overnight temperature, probably warming up to just around zero when we got up.
Robert